That is exactly what mystery shows are about, though. Mystery shows aren't about TRICKING the audience, they're about giving the audience evidence and allowing them to come to a conclusion at roughly the same time as the detective. BAD mystery shows are the ones that go "LOL WE TRICKED YOU".
They've been giving evidence pointing towards Kang since the start. It requires extra reading to fully get it but they can easily tie it all together in the show itself at the end. Having the big bad just being another Loki a) would be incredibly cliched and obvious and b) wouldn't make any sense. Loki isn't powerful enough or wouldn't have mastery enough to have existed before Time and then created the TVA / the monster that defends where he stays. If it was Loki as well he wouldn't leave a monster that could be enchanted to defend him since he would know that his variants would inevitably end up coming after him. It just doesn't make any sense to me at all.