Who said they tried to put a hard cap on Moscow at some lower number? Moscow grew to 8mln precisely because of government policies. Had they no limits, it could have been 2x that.
And yes, that policy is definitely baiting ex-USSR countries in the ass nowadays.
The fun thing with autocrats, they want everybody to be on short leash, on the distance of calling before eyes or local phone call.
So naturally, when they say they want to cap population of main cities - doesn't mean they're ready to delegate anything essential to 2nd tier cities! And they consider many many things essential being control freaks, so that's where you get additional growth.
E.g. in the USSR, the main filmmakers were in Moscow and SPb. Could you relocate them to e.g. Sochi as it happened with Hollywood in the USA? Sure, but would you be able to hold them as tightly? Hence they stay.
And yes, that policy is definitely baiting ex-USSR countries in the ass nowadays.