Saitama was definitely holding back against Boros, but i don't think he was holding back the way it's mentioned here.
What was implied after the Saitama vs Boros battle is that Saitama didn't use his strongest moves against Boros.
The strongest move he used if i recall correct was called "serious serious", so as far as Saitama is concerned, yes he was holding back since he probably has stronger punches, but comparatively to all other antagonists, he was pushed much more by Boros than anyone else, Orochi included, because he was the first to to survive his normal punches, and requiring a "serious serious" punch to be finished, which is why Saitama said "it is almost like a real fight" even before their all out assault and later said he is the strongest he faced.
Basically, Boros was at a level where Saitama could almost fight for real, something no one else accomplished since a normal punch was enough to one shot them.
What was implied after the Saitama vs Boros battle is that Saitama didn't use his strongest moves against Boros.
The strongest move he used if i recall correct was called "serious serious", so as far as Saitama is concerned, yes he was holding back since he probably has stronger punches, but comparatively to all other antagonists, he was pushed much more by Boros than anyone else, Orochi included, because he was the first to to survive his normal punches, and requiring a "serious serious" punch to be finished, which is why Saitama said "it is almost like a real fight" even before their all out assault and later said he is the strongest he faced.
Basically, Boros was at a level where Saitama could almost fight for real, something no one else accomplished since a normal punch was enough to one shot them.