Its ODA who has given Godfy's world class battle with Kaido who could have taken G5 form as much as he needed until he defeated Kaido and the same ODA writes luffy in egghead out of G5 for god knows how many times.
His previous statement is proof he himself does not know how to contain Godfy. He could have made kizaru stronger and make him knockout luffy but no.....Luffy will get himself knocked out.
Gomu gomu no fusen mode
Gomu gomu no baka mode
These are one of the many instances when he is not serious even when stakes are high.
we know how serious is luffy in G5 mode
With your (mis)interpretation of this quote, I guess you believe Luffy could've easily beaten Katakuri at the start of WCI, and Kaidou at the beginning of Wano. He just wasn't serious, and Oda had to "nerf" him, because if he didn't he would've just defeated the arc's villain immediately!
Nice that you conveniently neglect the fact that Luffy had to eat tonnes of food between G5. And yep, he could've made Kizaru stronger, but he didn't need to, he already beat Luffy as is.
You can pick your poison, either Luffy got nerfed this arc and therefore lost over and over, or he wasn't serious when everyone was dying around him which makes him a terrible person.