It can be helpful to look at the context on this page. Just a few paragraphs down, Deloria writes:
Behind each successful man stands a woman and behind each policy and program with which Indians are plagued, if traced completely back to its origin, stands the anthropologist.
The implication is that, when corporations or governments enact policies that hurt Indigenous communities, they usually look to anthropologists for evidence to back them up. If one stopped reading here, one might conclude that the main problem with anthropologists is that, because they are non-Native and carry with them the biases of the colonizer, they tend to provide misleading data to governments. Better data would lead to better policy-making and better treatment of Indigenous communities.
Deloria, however, seems to have something even more complicated than this in mind. In the very next paragraph, he writes:
The fundamental thesis of the anthropologist is that people are objects for observation, people are then considered objects for experimentation, for manipulation, and for eventual extinction. The anthropologist thus furnishes the justification for treating Indian people like so many chessmen available for anyone to play with.
The problem with anthropolists is not just the content of what they produce. It is the form and the method that they use. One anthropologist might write a report that suggests that the best way for the government to solve a clear water shortage in an Indigenous community would be policy X. Another anthropologist might write another report that suggests that actually the best solution is policy Y. Deloria is not just saying that anthropologists, as outsiders to Native communities, are unlikely to know whether policy X or policy Y is better—though that is definitely true, too. The fact is that even the way that they have set up the debate between policy X and policy Y in the first place is flawed. The assumption built into this debate is that non-Natives, if only they have the right data, can decide what is best for Native communities. This assumption is fundamentally colonial. It is, moreover, a direct consequence of the practice of anthropology itself. Thus, it is not merely what anthropologists say that is fundamentally genocidal in character. It is the fact that the discipline of anthropology even exists in the first place!