Eh... Kind of? You're right it wasn't exactly clear that he had been doing martial arts, but at the same time Mike was much quicker to want to throw down with Axel than Ian was at the club. Even a Chad Ian waited for Axel to get physical before Ian started threatening back, Mike jumped to a fight as first resort. I suspect the plan was for Mike to have always been in the MMA tournament, but Eva decided to introduce Mike to Ian a bit sooner with the "special guest appearance" in the opening round of the tournament.Just to add to this, before Mike was randomly added to the MMA update retroactively I wouldn't have said it was a thing we could have observed with him either. I doubt Eva will add yet another male character to the tournament at this point, but we should probably never say never when it comes to this thing. "Fighting (or beating up) Axel at the tournament" is an often repeated request.
This is one of those instances where, EK will likely add a line or two of dialogue to a Mike / Lena conversation earlier about how he's recently started training, or Ivy dropping it in an off handed comment that the DJ at her club has started MMA during a pole dancing class or whatever. EK does this often, going back to much earlier chapters and changing dialogues slightly, or adding more context for foreshadowing purposes. Usually it's overlooked by members of this forum because most of us don't really go back and play through the early chapters of the game everytime there is an update.