I agree with what everyone else says. We as readers want to see Natsu, in this last stretch of Fairy Tail, to be a lot more powerful after his time-skip. So, the opponent is hyped.
I
hate when people bring up Bluenote vs. Gildarts in a debate. It's like quoting wikipedia for an important school paper.
First of all, let's establish that Gildarts took out Bluenote in
one named attack at the end.
1) When Bluenote was first introduced, we as readers had to know a) who is this guy b) what are his powers. Hiro wrote Gildarts to fight Bluenote in a small skirmish first, not because they were "equal in power" but as a writing point so that we'd know who the hell this guy was. Imagine how underwhelming it would be to see Bluenote get one-shotted off the bat -- it would be Natsu vs. "God" (Beast) of War underwhelming. This is analogous to Hades vs. Laxus. Hades was clearly above Laxus, but allowed Laxus to fight around a bit because that is what experienced fighters do. They do not underestimate a new enemy no matter who it is.
2) Hiro had to nerf Gildarts with that tenrou tree thing. Gildarts lost his magic and was stomped on for a good while, while laying there like a magikarp out of water. This was probably to prevent Gildarts from having a reason to show up to fight against Hades later - he'd be too exhausted from taking mortal blows (similar to Jellal being slashed by Erza's sword in mortal form in ToH) to get up.
3) When all is said and done, Gildarts destroyed Bluenote in one
named attack. Crushing Evil Spread the Truth: Empyreon. The purpose of Natsu one-shotting bluenote was for us to see that Natsu (like Laxus and Jura) had entered Gildarts' level. This again can be backed up by the panel of Natsu and Gildarts' having their faces shown side by side as shown below:
So Gildarts was obviously very down-played for the sake of plot. He's been kept out of every arc simply because he can defeat the final boss (likely) and instead would be reduced to fighting fodder with Makarov to allow Natsu & co. to be relevant.
As for your "hiro stated this" thing, that was a department among the àapublishers, not Hiro himself, though I'm sure he himself did not care.