What I don't understand is why Syrian government militias are vacating the areas they control. Maybe they're saving themselves till reinforcements arrive.

Mercouris seems to think (today) that it was an intelligence failure. Syrian forces have only heard about it 2 days prior, it seems, and melted away since they didn't have the resources. You are absolutely right that the reinforcement is on it's way .. apparently it will arrive in 3 days.

No doubt this was cooked up between Izzies and Septics in conjunction with Erdogan .. who is miffed that Assad hasn't talked to him about Idlib. Putin just might be furious if Erdogan hasn't talked to him about it 2 days ago when they had contact with each other. Etc.