I suspect the reason Putin did this was to light a fire under the Brits, if not the EU, that their abrogation of military might leaves them wide open.
His purpose is to convince the assembled that they need a military, because if they build one, it will counterbalance the current big advantage, even if just a little, of the US.
It is a subtle thing, but a smart one, from his point of view. Lots of scenarios agree with him.
Perhaps.....Putin is a lot of things but stupid is NOT one of them.