I have been wondering whether what might have happened is: a US/UK/Fr attack with some, small, loss of missiles to the most modern SAA SAM systems. At the same time, the older SAA were fired unguided for reasons of morale (akin to heavy AA directed at Zeppelins which the guns could not reach). This because the Syrians and Russians accepted that most Syrian SAM couldn't achieve much and didn't want the escalation of Ru SAM being used. IF S-300 are now being supplied to Syria, it may be to ensure that a future US or Israeli attack can be deterred (unlikely) or at least come at some cost of resources. It may also be why RuAF bases are, apparently, being better defended: it might have been thought that, while the RuAF could knock down some incoming missiles, the base itself was inadequately defended. All waffle and speculation oc but I don't think a future attack will be as easy as the last.
If we accept that they did launch at least some missile without guidance, they may have done so under the assumption that the cruise missile attack could have been the first phase of a multi-pronged attack which included manned aircraft. Recall that one of the Russian press releases mentioned that guided bombs were used in addition to cruise missiles. This doesn't mean that they were used, but it does show that this aspect was under consideration by the Syrians and Russians.
Now consider a situation where cruise missiles were the first wave of an attack which would see follow-up by manned aircraft after the cruise missiles had struck. There would be extensive electronic warfare being used by the Americans, and indeed something along those lines was mentioned by them. This could have added to the difficulty of picking out incoming aircraft amongst the background noise. The Syrians may have left their guidance radars turned off to avoid pointing themselves out as targets. They may have then launched some missiles without guidance in order to try to provoke a reaction from what they assumed may be an incoming wave of aircraft. That reaction could include either aborting the attack, or doing something which would cause the incoming aircraft to reveal themselves and make turning on the guidance radars worthwhile for a follow up volley of air defence missiles.
Given how the incident was reported by the Americans, it is possible that the Americans had some additional actions planned and were very disappointed that the Syrians didn't play according to the American script.
The above is only speculation, but only someone who is very familiar with current Syrian and Russian air defence policy, doctrine, and training could really say for sure whether it reflects the situation as it existed there at that time. Keep in mind that the standard western views of the "proper" way to conduct air defence start from the assumption of complete control of the skies and so can't be relied upon to tell us how things look from the opposite perspective.