It's half true, or parts of it are true but are being used in a selective way to support some pretty racist ideas.
Even the site itself is contradictory. They claim Indians are racists, but put up one link after another showing intermarriage and acceptance? And put up a link showing the dialog between Black and tribal leaders.
And this?
"Having ignored the passage of the 13th Amendment to the United States Constitution,"
Ignored? Indian nations were legally separate nations at the time, though "domestic dependent nations" according to the Supreme Court. The 13th Amendment could no more legally free slaves among the Five Tribes than it could in, say, Brazil. That's why treaties had to be enacted.
"the tribes, chose to embrace sovereignty to keep the slaves in bondage."
Nonsense, they believed in sovereignty long before Europeans ever got here. A great many Indians sympathized with abolitionism and recognized that Southern white racists were responsible for the Trail of Tears. Also:
"The Seminoles provide an exception to the pattern of race relations that developed among southern Indians in the early nineteenth century. When the Red Stick Creeks joined the Seminoles in Florida following the Creek War (1813-14), they took many slaves with them. Together with runaway slaves from white plantations, these African Americans maintained a semiautonomous existence, with their own towns and leaders, and merely offered tribute to the Seminoles...By separating themselves from the "civilization" program and American thought in the early nineteenth century, the Seminoles apparently did not imbibe the racism that increasingly influenced the actions of other southern Indians."
http://college.hmco.com/history/readerscomp/naind/html/na_036400_slavery.htmBack to the passage they showed to you.
"The treaty, signed in 1866 by the same tribes that had formed an alliance with the Southern Confederacy"
Take a look at a map. OK was surrounded on 3 sides by the Confederacy. There were few federal troops in the area, and many well armed Confederate ones. Most felt they should make alliances with the Confederacy largely out of fear. At the first chance, all Five Tribes switched sides.
A few, mostly Indian plantation owning elites, did identify with white racists and also rejected their tribal heritage.
Even the way they phrased the question to you, "you people", well, they sound like Indian haters. And they sell and promote Kevin Mulroy's book and Mulroy is himself a (white) Indian hater.
There's been a push by some in the Black community, with some who are themselves racists and some others who are not, to try to get reparations from Indian tribes, try to force themselves onto tribal rolls, etc. There are also two main Freedmen descendants activist groups out there, one which is legit and another which is very unethical and has ties to Black Supremacist groups like the Nation of Islam and Nuage frauds like the Nuwaubians and Wash itaws. I don't know which of the two this site is associated with.