I'm not really sure why people are claiming bullshit or retcons on all of these developments when there haven't really been any big inconsistencies thus far. It's just that a lot of silly gags in early chapters were actually misdirects and foreshadowing for revealing Mirio and Iver's true strength/potential.
The author even specifically calls out the things that were never explicitly said and only misleadingly implied, so that readers can go back to check for themselves, like assuming that Mirio was weak just because he was inexperienced as a Hero. In particular, the reveal that at least part of the actual reason Mirio wasn't mentally corrupted by the Holy Sword (previously speculated to be because of the Power of Love) was because he was too fast and strong and made it to Iver's castle absurdly fast, is pretty well done.
I guess they just don't want to admit that the stupid ecchi manga was building up to an actual story this whole time?