They've been skipping around this issue ever since Rimuru became a demon lord, and I vaguely recall that I talked about it at some point, but at this point nothing should surprise Rimuru at all because of Raphael.
Raphael's supercomputer-like abilities should be enough to solve any problem by sheer brute force but eh let's pretend that Rimuru doesn't have that power sometimes to make things more interesting
Clayman mentioned Kazalim at one point during their fight, but based on a previous episode this season it seemed that Rimuru had forgotten that name, even if he and the other demon lords had talked about it as the previous demon lord that Leon replaced. I mean, wouldn't Rimuru had at least tasked one of his minions to research this Kazalim guy whoever he was?
There has also been at least 3 times previously where Raphael and Rimuru had agreed on who has been pulling the strings behind the scenes, meaning they know who it is, but has refrained from blurting it out loud for some reason, probably plot related
They haven't revealed it yet since going to that source will definitely take out all the tension with Hinata and Luminus and things would've been resolved without a fight.
No idea if I'm correct or whatever, these are just my 2 cents, I'm even letting all these slide since Rimuru acting dumb like this does lead to interesting stuff happening