I don't see how indigenous movements using a name that was originally used by only few peoples puts the whole of indigenous peoples in a monolith.

This reply was actually pretty good:

This is something every pan-racial (for lack of a better term) movement will run into. Every movement needs its foundational myths (in the folkloric sense, not the "stories that are false" sense) and common beliefs to function. When you're trying to unite disparate peoples whose main reason for joining together is to increase political/bargaining power, you're going to have to either choose to prioritize certain narratives over others or create new narratives whole cloth. Since legitimacy tends to come from either time or power and these movements lack power, most groups are going to pick the first option (in reality, a mix between the two options). Due to the relative prominence of the Eastern Woodlands in both American/Canadian history and members of tribes from that region in the creation of pan-Indianism, narratives from those tribes are going to be prioritized. And other people are going to go along with it, because joining with the movement is more beneficial than fighting it, especially for something as minor as this.

Even if they are right (and likewise I’m too ignorant on this to know) this comes across as concern trolling given the context. If they were making this criticism to the activists as another activist it could be a completely reasonable criticism but theyre making the comment in an r/neoliberal thread where theyre trying to make themselves feel better about being the beneficiaries of colonialism.