Unless there is something I’m missing the above isn’t true at all. They recognize Native Americans, Alaskans, Pacific Islanders, and Hawaiians as indigenous movements and call for full sovereignty of those groups:

https://frso.org/main-documents/immediate-demands-for-u-s-colonies-indigenous-peoples-and-oppressed-nationalities/

For the statement rejection of settler-colonialism, their position is (or at least the head theorist appears to say) that America is in a state of imperialist monopoly capital which is derived and developed from settler-colonialism. This development has its own sets of contradictions and characteristics, some deprived from settler-colonial basis and others from the transition to monopoly capital in that context, however due to being at the stage of capital development it is not the case in which the multinational working class of the U.S. merely has revolutionary potential in its oppressed nationalities as white supremacist ideology under monopoly capitalism has new characteristics that make it harmful in some aspects to the white working class and this there is a basis of exploitation for them to have revolutionary potential.

In essence the U.S. is monopoly capitalism with settler-colonial characteristics.

The article people use for saying they reject settler-colonialism is the following one:

https://fightbacknews.org/articles/red-theory-against-sakai-settler-colonialism-and-national-question-us

I think your initial statement was closer to correct unless there is some articles I missed or cadre only lines of ideology FRSO holds that the above user was referring to.