Probably a deal with the Russians.
It would have been more interesting if the Russians had tried to shoot down the missiles.
There would then be a better idea of the quality of Russian systems.
But thats a risk to the Russians, it benefits them to hide both their qualities and their deficiencies. If they are bluffing, they can continue to do so. If they aren’t bluffing it hampers their rivals in the R&D leapfrog game.
Russia doesn’t care about Assad, per se.
If he could be removed with Russian interests in Syria being protected, they could work with that.