Good point. I didn't think much about the question, and it should have been obvious that the hypothesis of him simulating honesty is not strictly falsifiable by relying solely on his words.

Ok, new possibility for falsification: before SIAI was founded, a third party offered him a job in AI research that was just as interesting and brought at least as many assorted perks, but he refused because he genuinely thought FAI research was more important. Or for that matter any other scenario under which founding SIAI constituted a net sacrifice for Eliezer when not counting the benefit of potentially averting armageddon.

Quite a bit harder to produce, but that's par for the course with Xanatos-style conspiracy theories.