Things like this can happen, especially if, like you guys said, the SAA is made up mostly with militias. They left behind a good number of equipment, that comes in handy for the Jihadists as they have literally an unlimited manpower pool.
Also the Russian Air Force seems to be quite passive here, perhaps not wanting to hit nearby SAA units in this chaotic battle... However, the Jihadist offensive was pretty strong and determined, it would be hard to contain. Despite me hating these animals, they're formidable soldiers because they couldn't care less if they die, relentless.
Being an SAA soldier must be really tough, relentless enemy not willing to spare your life.
The apparent corridor the Jihadists have opened is quite small, some say 2km wide. That could be easily pounded by artillery and airpower, but the Russian assets are limited leaving only the Syrian Artillery and the Syrian Airforce, which at this stage, with limited technology, can only do this much...