I mean, if you say "I totally killed that guy" in public I think the prosecution would be able to bring it up at the murder trial.
I hope Trump's lawyers all did a collective facepalm when they heard him say that.
This is why getting Trump on the stand is so imperative. He basically can't not incriminate himself. And he's such a fragile snowflake I think a simple matter of requisite media talking heads saying "Trump is a coward for not taking the stand, only guilty people plead the fifth, why doesn't he speak?" would be enough to goad him into doing it, despite what I'm sure are constant reminders from his lawyers that "the one thing you do not do is take the stand."
Because they know as well as the rest of us that if he does, it's game over for him.