Fair, if we ignore the "can"
It's easier to understand of you link point 1 and 2 btw. Why people underrated Luffy (beside people who hate him as a character but that's a different thing)
I think his interactions with Kizaru made the downplay for him skyrocket
Him not being able to really tag Kizaru or possibly even see him started a huge downplay train, which I find weird, because Kizaru is like the fastest character in the verse basically. Not sure how it's bad for Luffy to struggle to keep up with him.
And even with Kizaru taking Star Gun without a lot of damage, that's nothing new. Sakazuki took a quake fist from a VERY pissed Sickbeard off guard and while it very clearly injured him, he still withstood it and took off about 40% of his head. Not to mention other impressive endurance feats from top tiers like Blackbeard and Luffy himself.
It's not surprising that Kizaru could take Star Gun without being heavily damaged, he wouldn't be a top tier if he fell to a move like that immediately.
So I'm not sure why people downplay Luffy for it, it seems more like it was just hype for Kizaru.