It 'proves' nothing of the kind. One can disapprove, want someone gone for his decisions and actions while decrying the naked obstruction that is inherent in firing someone for a stated reason, and then revealing the real reason for the firing in a public interview.

Trump doesn't like the Russian investigation in which he may or may not be a 'subject'. Doing something about it is obstruction of justice.

If Comey said Trump was not the subject of an investigation, 3 times no less, then why would Trump fire him?

Trump is f*cked and he knows it.