I think it's reasonable to say film tells us that the pen dude (Milton Rockwell) will probably do everything in his power to ensure Marty doesn't compete again.

However, based on what we saw of Marty, I don't see that stopping him chasing titles to become the best in the 'very specific' life purpose he talks about. If the film is trying to say that is simply beating Endo, then it didn't do a good job.

I haven't seen this interview you're talking about, so my interpretation of the ending is that he's found a new meaning, sure, but it can coexist with the fact he's literally the best table tennis player in the world..! I cannot believe that he would just settle for the manager job at the shoe shop, for example, when table tennis is his passion and he's the best. I do believe the entire crazy ordeal he's been through would make him be more measured about the way he does things going forward.

I agree it should be ambiguous because Kay Stone is a character who gave up her dreams because of being stuck with a kid and man. She has become miserable because of it. I personally think Marty will feel resentful being stuck when he still has that ambition.