Another possibility is that someone found a software vulnerability and inserted a virus into the C-cubed-I system causing them to not fire anything. (The Israelis did this about eight or nine years ago.) The Russians certainly would not say anything as it would confirm a system vulnerability to countermeasures. We would not say anything either.
So no missiles are fired and no one admits anything. Sounds like a system failure but could be much more specific than “lousy system”.
Based on what I read in the European media after the attack it appears that the Russians did not activate or turn on the S-400 system. The claims by some that this missile system is so advanced that it could clear the skies of aircraft is simply stupid in the extreme. Throughout the history of warfare for every great thing there is a countermeasure - often surprisingly cheap.
I imagine the future (in the short term) of anti-aircraft suppression lies in drones and stealth technology. There is no way to hide the transmission of radar - it announces its presence loudly and it makes the most essential part of the system very vulnerable. Once it activates if there are drones in the area the radar is toast. There is also no evidence that the S-400 reliably detects our stealth aircraft.
Despite the bluster from Putin, there was no upside to them employing the S-400 to protect facilities that Assad had already evacuated so they can continue to tout their system and we can continue to speculate on its effectiveness, but without a doubt drones have changed and will change the threat and countermeasure game in ways we cannot imagine.