If C# they'd have been using the MS authored GC. It was probably a custom GC for C++ heaps. Quite a few games do this, it's a smart productivity hack. Unreal uses a simple form of mark/sweep GC for its C++ game state heap.

https://unrealcommunity.wiki/garbage-collection-36d1da

It works fine because most RAM in games is consumed by assets that don't need to be scanned.