Soldiers fighting along side different branches , like armour , artillery and AT would gain at least some form of familiarity with the common weapon systems used , not to the same standard as a trained crew , but if it came down to it they would not just ignore these weapons if they were at all serviceable and had ammunition .
The equipment that was captured a minute ago is very likely to be either damaged in the fighting or disabled by the crew. Anyway, how familiar is a regular infantryman with repairing a gun's hydraulic recoil mechanism or finding, mounting and sighting in an optical sight on a division level AT-gun? Does he just abandon his duties within his squad and gets a couple of his mates and forms a crew? Are they actually willing to risk it and try to use a weapon system that they only seen from a distance? Is he familiar with repairing, maintaining and operating a T-34 just because his company was fighting alongside some StuGs? How is fighting as a grunt in a battle where armor, arty, and AT were used make's a man any more familiar with their operation? I mean you've got your own tasks and situation to focus on, how is briefly observing a platoon of panzers doing their own thing going to help you? Yes, you don't need a PhD to fight in a tank, and anyone will eventually figure out the basics, but it should take quite some time and likely will take place when there is a prolonged lull in the fighting.
Sorry for the rant.
I agree that in some situations they should find a use for the undamaged abandoned battalion level heavy weapons (such as hmg's, atg's, mt's, ig's), but i expect them to perform much worse compared to a trained crew. In the case of the topic starter you could imagine the crew disabling the gun before abandoning it, this practice is mentioned in the contemporary accounts. They could remove the AT-gun's sight or damage the recoil mechanism and remove and disassemble the bolts of the AT-rifles. As far as i know there is no such mechanics in the game at the moment, but one can dream.