He's a "futurist", who's explicit job is to make hand wavey, big, shocking claims. Historically, futurists have a less than random chance at being right about the future.
Also I would not consider him an expert in anything ML, though he probably has enough of the underlying math to get some things.
Kaku is a media sensationalist; that class of person has a terrible track record. Futurists, that is, people like Aasimov, have an ok track record at predicting the future. Interestingly, early futurist predictions about digital tech were pretty good. For "physical" goods? Not so much. A plausible explanation of this discrepancy is that they didn't predict the breakdown in the historial trend of increasing energy usage. See the Henry Adams curve.
I'm saying explicitly that the words that come out of Mr Kaku's mouth are noise, not signal. If he is right about something, it is in the way a broken clock is occasionally right. I agree with most of what he is saying personally, but that's not relevant.
string field theory is a a "field" of physics which is entirely theoretical and has no experiment data to back it up....
hard to imagine how even the top person in that field would be a top physicist given the large number of top physicists who are doing useful stuff like LIGO, CERN, etc.
Also I would not consider him an expert in anything ML, though he probably has enough of the underlying math to get some things.