Trump was not under investigation. So what? Flynn was and Comey offered evidence that, when combined with the President’s words (saying he fired Comey becaus of the Russia thing) and actions (firing Comey), is strong evidence of obstruction of justice. Enough to convict? Probably not because mens rea is always hard to prove.
I don’t know exactly what Comey said on collusion, but I’m not at all surprised that there was no evidence of collusion at the time Comey was fired. That doesn’t mean there won’t be at some point (although I’m highly, highly skeptical that we’ll ever see evidence of collusion). That said, it’s amusing that Trump straight up calls Comey a liar while Republicans simultaneously hold up Comey’s testimony of “no collusion” as the gospel truth. Comey’s a liar unless he says something that benefits Trump, then his word is the gospel and the case is closed.
Comey did highlight that perhaps the dumbest thing Trump has done was tweet about a secret tape. If the tapes exist, it’s hard to understand why Trump wouldn’t release them. If they don’t exist, Trump is exposed as lying in a dispute that largely comes down to “he said, she said.”