Why would he lie about Boros being impressive when Boros can't even hear him? He said it in space and to Genos. He was lying about it being a close fight and Saitama himself having a good time, not about Boros being a genuinely powerful opponent.
Saitama says directly that he can't tell how strong most people are due to the gap between them. Like how he has no idea how strong Gouketsu was, because to Saitama, Gouketsu was fodder. Saitama generally only calls people strong when they can impressive. Another main example is with Awakened Garou. After Garou attacks him, Saitama specifically says that he can tell Garou is strong, immediately noting that Garou isn't like most other opponents. With Orochi, Saitama never addressed his strength even remotely. Hell, in the original version it was somewhat similar, he didn't treat Orochi like a joke, but Saitama said that he doesn't care about Orochi's title nor how strong Orochi thinks he is, and in the end he still kills him effortlessly without regarding Orochi as a strong opponent. I doubt Saitama could've used the same old Normal Punch to cancel out CSRC. I'm not saying the Serious Punch was his full power or even necessarily close to it, but Saitama did not view Boros as another random opponent, that much is undeniable.