That makes sense in terms of cheese making, however I've never come across rennet being listed on any protein I've ever brought, unlike cheese, where it's obviously stated microbial culture or rennet.

Whey protein is produced in a lab process and it's hard to know the source in exact detail to be honest.

To make matters even more complicated, rennet can be derived from a vegetarian source too. In fact, when I read up a little bit, it seems vegetarian sources of rennet are more common than animal sources due to the quantity needed of stomachs of young calves for example (not necessarily pork either).