If theymos really said this, it is easy to guess his reasoning. You don't need to ask him.
1- Staff endorsing mixers might get the forum in trouble.
2- Mixers can't guarantee your privacy even though they say they do. That's possibly false advertisement.
But these points are valid for other services:
- What guarantee is there that a casino doesn't close and scam all its users?
- What guarantee is there for any service that is advertised to state that it has privacy, and not actually does not?
I can even understand why @theymos might have asked for this. Whether he really did, I don't know.
But those are not the reasons for sure.
The moderation of the forum, can be considered employees, so everyone should avoid situations that put the forum in question.
Anyway, if @theymos really made this kind of request, he must have strong reasons. And I trust him, and his good management of the forum. So we may not understand right now, but at some point, I believe this will clear up.