This is only a partial summary of the entire discourse that descendants of trans-atlantic slave trade are an "indigenous" group, and specifically indigenous to the areas they were forced to labor and toil, but here is a snippet. Think, BIPOC as an acronym (as opposed to the modern generalization of POC - it used to (~1960s & prior) primarily only refer to black people). The argument relies on defining settler-colonialism, or more specifically, that it creates a class relationship of settler and indigenous -- without this force there is no indigeneity. While black slaves were the labor force used to do the "settling" they were treated as tools and not the settling agent themselves. That's the more ancient history of it, but regarding modern post-reconstruction there's also analysis of black communities being the target of settlement(effectively colonization) by whites in the same patterns used to control and eliminate native americans but adapted to new eras with new technologies - think jim crow, deception regarding health/forced sterilization, redlining, Tulsa massacre, MOVE bombing. And even today it's directly paralleled in policing, school-to-prison pipeline, underdeveloped and underserved communities, missing women&children.

To be clear this (victims of transatlantic slave trade and their descendants are indigenous to where they were eventually taken) is in opposition to the idea that they are indigenous to Africa. Back to more antiquated history, "back to africa" at a very early iteration: American Colonization Society which was founded by white slaveowners that wanted to deport freeborn black people to protect the institution of slavery. The ACS did end up reproducing settler-colonial structures in Liberia, but not without heavy opposition from freed black folk who saw through the dressings of philanthropy for the underlying motives of exclusion and expulsion. Many of the "colonizers" for the ACS were coerced to participation, their freedom from slavery but in exchange you must get on that boat and go to a land you have never seen or known and you don't know a lick of local language or customs (the latter less important because you're there to colonize). For those outside of reach of coercion to join -- can you just imagine being a newly freed slave, seeing a white abolitionist (who found common ground with a slaveowner) tout that they wanted to find "your kind" a designated place free of racism, but it's not here, they didn't want to create that space, didn't think they could be actionable actors to affect racist attitudes in white society. The extent of the opposition to ACS was such that freedpeoples self-referential literature/social groups even dropped "African Americans" to "Colored Americans" to indicate that they belong on this land.

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the more finnicky details about specific land repatriation (specifically in north america anyway) aside I also think it's important that landback movements also take into account progression and not regression, and on that front I agree that establishment of DOTP is crucial. Unfortunately a lot of the "landback" ""activism" (instagram infographic-derivative variety) tends to not have this milestone anywhere near their goals. Liberalism(includes anarchists) reigns in this clade, which is how you might see an online "landback activist" who doesn't know a single name of an indigenous ethnic group or tribe, at least outside of North America or Europe, but loves to say such strange and uncritical things like "landback for Ukraine (pro-NATO)" or "landback for Tibet/Xinjiang/Taiwan (anti-PRC)"