It's most likely a giant C++ server or set of servers. It's not really that surprising that old code is in there. One way releases are handled in these kinds of places is to roll out new features on some kind of external flag system, perhaps shared memory flags or some other mechanism. This way if something goes wrong with that one feature, you can disable it by flipping the switch, rather than having to back out the software, which would likely cause downtime during market hours, a major problem.

What sometimes happens when you want to decommission features is you just turn the flags off rather than remove the code. There's an obvious allure to this as you have already tested the on/off functionality of the switch when you did the original roll-out so you can avoid having to test whether you have removed the code correctly. It sounds like in this case they removed the code and repurposed the switch that disabled said code (it may really be a shared memory system and they are running out of flags), but they fucked it up. The old code was still there on some servers and the switch was turned back in intent to enable the new feature it was re-purposed for, re-enabling this old code.

It was a flag that was re-purposed. If the code is never deleted and the flag's use is discontinued then the code never runs. However, it seems that the flag was reused years later causing the old code to be inadvertently reactivated. It has nothing to do with the language it was written in.

Why would the language matter? Dormant code is the same in any language.