@IrregularPerson No, Clayman is a Demon Lord, an unawakened "Demon Lord Seed", just like what the Orc Lord evolved into when he became the Orc Disaster. We know that Clayman is one, because someone needs to be one to even be able to make use of souls to become a True Demon Lord to begin with.
As explained in chapter 60, Rimuru himself reached that level when he absorbed the Orc Disaster, but because the information for Laplace and Clayman got cut off, they never knew about the Orc Lord evolving into an Orc Disaster, so they probably just assumed from that that Rimuru was at around Gelmud's or the Orc Lord's pre-evolution level at best.
Then during the Charybdis fight, due to Charybdis' magic interference, no one's power could really be properly measured (plus it ended up being finished off by Milim in the end) and Rimuru never really lets out his full power otherwise, so Rimuru unintentionally duped Clayman's observation like that without even noticing, causing Clayman to just view Rimuru as only a "Majin" still all that time, that probably also caused Yuuki's and Laplace's information about Rimuru to be wrong. Yuuki only exchanged as single kick with Rimuru after all, so they can't really read him all that well.
In the end Hinata was still stronger than Rimuru at the time of the ambush, but Souei's presence and Rimuru's instant reaction of setting up an insurance ended up saving him savely, now with quite some intel of Hinata's abilities and the church's most high-level battle tactics like that Holy Field, then Rimuru got rid of the Farmus army and then Diablo even "accidently" redirected Razen's Nuclear Magic against Clayman's observer in the sky, and also got duped about by Myuran's death the moment Rimuru was aware of Clayman's actions, so Clayman's information warfare was a cluster of bad luck and overconfidence that resulted in this situation where Clayman becomes more and more blind to what's going on in Tempest due to losing two of his most stealthy agents.
Basically Rimuru got dragged into these plans likely because he seemed too weak (Clayman, Yuuki and Kazaream might have changed their minds otherwise and chosen a different target instead), but that fact also ended up saving Rimuru's and the other Tempestians' lives as Tempest at full power really was nothing to scoff at, especially if they started bringing back the Orc Army in the long run, Farmus would still lose eventually, they never had any chance of winning, but Yuuki's group and Clayman didn't know anything about that.
All this is information from the manga, btw.