It's not you who should use UDT, it's the world.

Do you mean "it's not only you", or "it's the world except you"? If it's the latter, it doesn't seem to make any sense. If it's the former, it doesn't seem to answer Eliezer's objection.

I mean the world as substrate, with "you" being implemented on the substrate of FAI. FAI runs UDT, you consist of FAI's decisions (even if in the sense of "influenced by", there seems to be no formal difference). The decisions are output of the strategy optimized for by UDT, two levels removed from running UDT themselves.

Do you mean FAI should use UDT, and humanity shouldn't?

Yes, in the sense that humanity runs on the FAI-substrate that uses UDT or something on the level of strategy-optimization anyway, but humanity itself is not about optimization.

By "stuff", do you mean "part of the state of the world"? And people do in some sense embody strategies (what they would do in different situations), so what do you mean by "people are not strategies"?

I suspect that people should be found in plays (what actually happens given the state of the world), not strategies (plans for every eventuality).