The central point is that under Corbyn and co there is a good chance that the conflict may never have happened, and whilst I am far from agreement with specific policies, I do think that the basic thrust of policies of disengagement, non-intervention and cessation of arms sales to terrorists sponsoring repressive regimes in the region seem to me to be generally positive things.
I'd suggest you have a read of Robin Yassin-Kassab's book, as mentioned above. He discusses in detail the reaction on the ground when Obama failed to act after his "red line" on chemical weaposn was breached. A massive sense of betrayal and being abandoned in the face of superior hostile forces. It tells the story of what this felt like from a Syrian's perspective, people fighting for the same rights that you and I enjoy every day. The reality of what people desire and experience may not fit into our preticked ethical boxes.
The problem is with the "we don't know what's going on' narrative is that's exactly the purpose of disinformation. It isn't to win an argument, it's to muddy the waters, to provide "supressive fire". Assad has been indulging in this since the beginning of the conflict, and such distortions are now greatly amplified by Russian state media. Corbyn repeats and buys into these narratives, when he casts doubt on who carried out the Khan Shikhoun massacare, as I've said several times. It's overwhelmingly clear that Assad did it, he's the only actor with Sarin in Syria. Syrian airspace is some of the most heavily monitored in the world, the US saw the planes go in, Assad and Putin's narratives don't match for a second, there's all the eye witness/victim testimony on the ground. The Times even published a story identifying the pilot (Mohammed Hausori IIRC). Corbyn's "well we need a full investigation" doesn't mention the fact that Russia has blocked condemnation of the attack via it's UN veto. As I said, this provides tacit cover for Assad. Corbyn seems to have a problem mentioning his name as his ideology drives him to pin every problem on the West.
That's the specifics. More generally - I'm not against non-intervention per se, but just against any ideologically driven formula being doled out regardless of the details of the situation. Foreign policy can save lives and i'd favour an ethically driven process of engagement rather than withdrawal, though the former requires a lot more effort and the applicaiton of intelligence. . I mentioned Mount Sinjar above - this is an instance where intervention clearly and unequivocally saved lives. Corbyn's formula applied here would have let thousands of people die.