It's a stalling tactic. He'll start attaching ridiculous conditions to his supposed agreement to testify that the cmte will have to refuse for the session to have any validity at all. Then he'll say they are the ones 'obstructing the truth' or 'wouldn't allow him to testify'. The real point, though, of anything he does is to stall until after the midterms and hope the whole thing goes away. And it surely will if the GOP regains the majority.
As far as self incrimination, the likelihood of him telling the plain truth is as likely as me leaping the Empire State Bldg. He'll lie, redirect, blame, obfuscate, and take the 5th until his testimony is rendered worthless. After all, that's the only thing he's truly 'competent' at.