Give me an honest answer. Do you actually think Mashima meant x792 Gildarts when he made that comparison? You recognize how much of a stretch it actually is? We can argue all we want. But Mashima probably didn't think about which version of Gildarts and what not. Like others said here, Mashima clearly doesn't care enough about FT to actually think through this
The thing is, I'm doubtful that Gildarts had any reason whatsoever to flex his MP, the last time he flexed his MP and went all out was during x792 against August (a vastly superior opponent). Laxus likely wouldn't know the extent of x793 Gildarts' MP, so would have to rely on the last time he's seen him go all out.
But that is speculation and trying to make sense of Laxus's statement as there would be no reason for him to know how much Gildarts' MP has grown unless Gild flexed his entire MP in front of or around Laxus, and Gild doesn't seem like the type of guy to randomly flex his MP for no reason, so what Laxus would have to go on would be during Alvarez. Just like how Erza didn't know just how much Laxus himself has grown. If Erza didn't know how much Laxus grew, is it truly a stretch that Laxus wouldn't know how much Gildarts has grown and made a comparison on the last time he flexed his MP?
I said the same in my previous post, that Mashima could just say that Gildarts was just so above everyone else in Alvarez (other than Serena, Dragon Irene, August, etc) that his MP is still a baffling thing to Laxus. Laxus would be making the comparison on the last time he felt GIldarts' full MP level, which could be during or after Alvarez. Trying to scale Gildarts now would also be heavy speculation. Did he grow so vastly above his peer Serena that he could handle him as easy as he did the Historia? Is he August tier? Did he not grow much stronger at all? Is he DG tier now? Etc. Realistically, yeah, Gild has grown much more powerful within that year, but would Laxus know just how much he has grown?
Historia Serena was so powerful that even Erza commented on his strength and got worried, and a casual Gildarts waltzed in and low diffed him.
You're right, Mashima doesn't give a damn about power-level consistency, someone can go from Erza tier to Juvia tier depending on what the plot demands. This is seen with Kiria where she went toe to toe with a serious Erza and held her own for a bit only to be stomped by Laxus. She was proven to have extremely high durability, took a beatdown from Laxus and kept getting back up until he used a Secret Art, but is one-shotted by Lucy's new attack despite being completely healthy while she just got done destroying Lucy.
He could be commenting on x793 Gildarts assuming Gildarts flexed his full MP around Laxus, assuming he was, then the BDSK should be vastly above Serena in the MP department.
Mashima likely did mean x793 Gildarts without considering the likelihood of Gild actually flexing his full MP around Laxus and the others for no reason.