I've just clocked. That power up for Naruto in the latest Boruto chapter isn't an ass pull. In fact, if we follow the events after the 4th Great Ninja War i.e. the novels it makes perfect sense. Lemme explain:

So you know how Naruto lost the truth seeking orbs after his fight with Sasuke? Well, that's largely the reason why he hasn't used his SO6P powers anymore in the manga (or an extremely limited version of them)

Then, in the novels it's been implied that Naruto managed to seal the second half of the 9 Tails into him in order for Kurama to be at one again. So obviously he was going to receive a power boost off of that

He was powerful already with 1 half of the Kyuubi but now woth both halves he's clearly gonna be stronger so Kurama giving Naruto one insane power boost at the end isn't deus ex machina or a plothole; it's been an implied foregone conclusion