This is 99% true.

While many believe that Thanksgiving Day is related to the mythical 1st Thanksgiving of the Pilgrims, that linkage wasn't actually made until later.

The national holiday we actually celebrate, which is annually at the end of November, is a celebration of the victories of the United States at Gettysburg and Vicksburg. These battles sealed the fate of the slave-owner rebellion, both by ensuring defeat was inevitable, but also removing any possibility that the British and French Empires were going to enter the war on the side of the Confederacy. Lincoln had much reason to give thanks that year.

"The year that is drawing towards its close, has been filled with the blessings of fruitful fields and healthful skies. To these bounties, which are so constantly enjoyed that we are prone to forget the source from which they come, others have been added, which are of so extraordinary a nature, that they cannot fail to penetrate and soften even the heart which is habitually insensible to the ever watchful providence of Almighty God. In the midst of a civil war of unequalled magnitude and severity, which has sometimes seemed to foreign States to invite and to provoke their aggression, peace has been preserved with all nations, order has been maintained, the laws have been respected and obeyed, and harmony has prevailed everywhere except in the theatre of military conflict; while that theatre has been greatly contracted by the advancing armies and navies of the Union. Needful diversions of wealth and of strength from the fields of peaceful industry to the national defence, have not arrested the plough, the shuttle, or the ship; the axe had enlarged the borders of our settlements, and the mines, as well of iron and coal as of the precious metals, have yielded even more abundantly than heretofore. Population has steadily increased, notwithstanding the waste that has been made in the camp, the siege and the battle-field; and the country, rejoicing in the consciousness of augmented strength and vigor, is permitted to expect continuance of years, with large increase of freedom. No human counsel hath devised nor hath any mortal hand worked out these great things. They are the gracious gifts of the Most High God, who, while dealing with us in anger for our sins, hath nevertheless remembered mercy. It has seemed to me fit and proper that they should be solemnly, reverently and gratefully acknowledged as with one heart and voice by the whole American people. I do therefore invite my fellow citizens in every part of the United States, and also those who are at sea and those who are sojourning in foreign lands, to set apart and observe the last Thursday of November next, as a day of Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens. And I recommend to them that while offering up the ascriptions justly due to Him for such singular deliverances and blessings, they do also, with humble penitence for our national perverseness and disobedience, commend to his tender care all those who have become widows, orphans, mourners or sufferers in the lamentable civil strife in which we are unavoidably engaged, and fervently implore the interposition of the Almighty Hand to heal the wounds of the nation and to restore it as soon as may be consistent with the Divine purposes to the full enjoyment of peace, harmony, tranquility and Union. In testimony whereof, I have hereunto set my hand, and caused the seal of the United States to be affixed. Done at the city of Washington, this third day of October, in the year of our Lord one thousand eight hundred and sixty-three, and of the independence of the United States the eighty-eighth." Proclamation of President Abraham Lincoln, October 3, 1863.

Obviously Lincoln had never heard of paragraphs.

The United States came out of the war so powerful, that it was able to tell France to get the hell out of Mexico, and Mexican rebel armies (led by a man of Indian ancestry, Benito Juarez) were able to defeat the French installed Emperor Maximillion only two years later. Our victory was Mexico's victory, and we got another holiday out of it, Cinco de Mayo, which is a celebration of an early Mexican victory against the French (before they lost the war and were occupied). After Reconstruction ended, and the South once again came to dominate American politics, it became politically incorrect to celebrate Union victory, so instead we celebrated victory over the people who lived here before our ancestors invaded (most of our ancestors anyway).

From 1863 to 1938, Thanksgiving was on the final Thursday of November. In 1939, the Great Depression was in full force. November that year had five Thursdays. Roosevelt decided to move the holiday to second to last Thursday of the month, in order to create a longer Christmas shopping season. Republicans (this is no joke) said this was an affront to the memory of Lincoln, and that year there were two Thanksgivings, "Democratic" Thanksgiving (or Franksgiving) and Republican Thanksgiving ... just in case anyone was under the illusion that intense partisan stupidity was a feature of our era alone. Texas, in an unusual display of bipartisanship, celebrated both. In 1942, Congress made Thanksgiving the fourth Thursday in November, and it still is.

So while there were occasional earlier days of Thanksgiving, even going back to Spanish colonization (1st Thanksgiving in what is now the U.S. was in San Agustín, La Florida, 1565), the real history of this national holiday is about war and shopping. And that's about as American as you can get.