I don't think it's possible to debunk this: you can't prove it, just like I can't prove I don't work for the NSA (so he's saying my signature may work for the NSA?).
This is VERY important to debunk because FatManTerra has an entire echo chamber on Twitter and also because he's telling them that "centralized mixers are very illegal".
I'll throw in a counter argument: privacy is a human right.
I only know "FatManTerra" from
this 4 Bitcoin bet, where he tried to
change the Bet terms afterwards. He's not someone who's opinion I value.