Russia recently tested a nuclear powered cruise missile. That's probably what Trump is actually referring to here. I'm not sure I disagree with him either.
Discussion
Russia tested something that worked?!
It's unclear how well it actually worked at this point. But they're clearly trying.
Maybe they tried a delivery system. Even for that, I have doubts. Testing is done for everyone to see, not in "secret", because it's a flex and nothing more.
But I strongly agree with you about Trump "showing how it's done".