Given how Trump's brain works like an old sofa, it seems more likely that he's recently been talking to someone who still calls it Leningrad, such as an older Russian with strong soviet-era ties/sympathies, maybe even someone who was born in Leningrad... en.wikipedia.org/wiki/Vladimi...
In fact, I would even go so far as to speculate that "If I got Moscow and Leningrad free, as part of the deal with Russia, the Fake News would say that I made a bad deal!" is a direct quote of a quip Putin made during their 'negotiations' and Trump's ego immediately filed for future use.
Which is to say, Putin is once again playing Trump like a fiddle. Playing up to his ego to convince him that whatever deal Putin wants him to make will be a masterful deal greatly favouring Trump.