It’s hard to draw sensible conclusions when governments now routinely lie through their teeth. I find the western claim of 100% success rate far more dubious than Russian’s claim of very high engagement rate, which itself also seems a little too good to be true. The reality is probably somewhere in between, and I find it far more believable for at least 1 missile to have been intercepted or simply malfunctioned and crash than for every single one to have it their targets.
It could well be that on the night, the Russians and Syrians were expecting a much bigger and comprehensive raid, and so were conserving their ammo since the sites targeted were of minimal or no military significance.
From the Russian POV, the attack could easily have been mistaken for a faint designed to make the defenders waste their missiles and reveal their locations, making them vulnerable to a follow up strike.
The S400 and A50 may well have detected massed western combat aircraft further out and assumed that was the main strike wave, and was saving their missiles for them.