Raoul did specifically say the king spent 20 years inside the "miniature world I created."
Maybe it's being kept intentionally vague wording-wise if it's actually an illusion vs a literal pocket dimension with different time, for a "big reveal" (that we all saw coming since we noticed the aging) next chapter. Or maybe calling it an illusion in the first place is a misinterpretation on the translator's part.
Either way my guess is the original de-aging was done by the actual nekomimi god, Raoul then killed them, and he'll reveal that next in a flashback. Because we don't have enough flashbacks yet, of course :^)
It just wouldn't make much sense for Raoul to actually enable the king to get off his life support machine (which implies actual de-aging was involved) just to pull this troll. It would have to be an illusion from the very start, with the king still on the machine- actually I wouldn't put trying that as a twist-on-a-twist above this author either.
20 years haven't passed at all in real life, why would someone even think that? Raoul did literally the same thing to capital citizens to test their souls, making them think 3 months have passed inside an illusion of a paradise while irl it was about an hour, and it was probably that long irl only because there were a lot of citizens and Raoul had to check each one. The king is a single entity.
It just wouldn't make much sense for Raoul to actually enable the king to get off his life support machine
He said it himself some chapters ago (during national mage arc), he really enjoys giving his enemies hope before crushing them completely. So it's perfectly in character for him. Although we lack information at the moment whether
1) Raoul himself gave the king new powers after intercepting the original god of war and taking his shape
2) Actual god did give king new powers, but Raoul killed the god soon after and learned of his appearance and will now be mocking king that he killed the god
3) Raoul infiltrated the gods themselves and sat amogus them during that meeting, there was never a catboy god.
4) King never received any powers at all, he is still old and decrepit while connected to the machine, and the illusion started there - he never even left the palace.