Where did the FSA get the US made TOW missile system? Probably not from us. But there are literally dozens of nations that use it, and it can’t have been too hard for someone to slip pretty fair numbers of the TOW system and some older missiles to the rebels. You could fit the whole thing in a car.
And you’ll notice it’s pretty dirt simple to assemble and operate (at least, during daylight, against a single stationary target).
Notice also the relatively long time of flight for the shot. The TOW is a fairly slow missile, with a time of flight of up to 23 seconds out to its maximum range.
And finally, notice also that the tank (my eyes are failing, I can’t tell if it’s a T-55 or a T-62) has plenty of secondary explosions in the aftermath. Tanks may be a steel box on treads, but they’re also packed with stuff that loves to burn.