The only currently known resistance to ivermectin and moxidectin is a shortened ERP - 4 weeks for ivermectin instead of 8, and 8-10 for moxidectin instead of 12, and that’s for strongyles.
I’ve never read anything about a bot resistance, to anything.
Moxidectin works for as long as it does, in part, because it is not as easily kicked out of the parasite cells, so works to kill things for longer. Or rather, in this case, paralyze to whatever extent is necessary to make them let go and be expelled.