Well to be fair, it does have some chymosin, and it does set milk. It's just that the chymosin/pepsin ratio isn't quite favorable to making hard aged cheese. That being said, I've made cheese from it before that was okay. Not amazing, but pretty decent, especially when using good milk. The term "rennet" can refer to any sorts of enzymes found in the abomasum, including chymosin and pepsin.
What happened is that the founder oft the modern Chr Hansen (named of all things, Chris Hansen) was clever and devised an approach to create more shelf life stable and better quality rennet extracted from abomasums. Since then there have been better methods devised, but that old method and style of manufacture is still in an old factory in NY (IIRC) where Junket is still made. There's nothing so wrong with the method, but nowadays, all the prime abomasum is used to create costly calf or kid rennet and sold at a premium. And when kids grow older, the enzyme ratio shifts in favor of pepsin. Heck, you can even extract a rennin/pepsin mix from pretty old calves, and in small amounts from adult animals. So in the end, you'll get an enzyme mix, but it's not really that great. The set is usually softer, and it doesn't age the same way as prime rennet.
