i'm actually having a pretty hard time seeing it as a joke referring to the tape scandal, tho i could see it as maybe a joke like "haha we told you that you got fired because you mouthed off confidential information inappropriately to the press so you better not do it again ;)"
either way it does come off as vaguely threatening, but i doubt it's grounds for any charges lol
After some thorough thought, I can see it potentially being interpreted that way, the only thing is, almost nobody is interpreting it that way and even unintentional witness tampering has a minimum sentence of 3 years. Even /r/the_donald, the most unabashedly pro-Annoying Orange place on the internet, thinks it's a threat. The only difference is they're okay with it.
I don't expect any charges because I don't see comey being tampered by it. In fact, according to one of his aides he's actually hoping there are tapes.