I think the point the GP is trying to make is that his arrest does not obligate him to testify in front of anyone: his "right to remain silent" and all that. Prior to being arrested, voluntarily testifying (or complying with a subpoena to do so) before Congress could have been useful to him if he could manipulate things well enough to get the heat turned down. (I don't think he'd actually be successful at that, but that's another story.)

Now that he's in jail, he may actually listen to the advice I'm sure his lawyer has been giving him all along: shut the hell up. And he's perfectly within his rights to do that. Even if he is hauled before Congress, he can simply assert his 5th Amendment rights after every question, and that might actually be the best move for him now.

Edit: re-reading the GP, I realize what you were objecting to, the "I'm pretty sure the house cannot make him testify if he is in custody" bit; I agree with you in that I don't think that's the case. Still, though, even if Congress can force him to testify, they can't actually force him to answer questions. They technically couldn't do that before, but now that he's been arrested it's likely even less in his best interests to talk.