He’s not retconning it. Just ignoring it. When asked what they considered canon when writing the films, JJ merely said the OT.
As for Kylo, I doubt his training has been that short. He’s probably had several years at least. Regardless, even Ezra is capable of constructing a fully functioning lightsaber so clearly there is some disparity there.
Like I said it's not solely JJ's movie. The story group will tell him what he can and can't do.
It's clearly something they learn early on, but with no one teaching them it might be harder to figure that out. That's what Dawn of the Jedi suggests anyway. Luke has a spare Lightsaber to break apart in that, but still can't figure it out. He just knows that the Crystal is somehow kept in place with the Force in the construction of the Saber.
Ignoring the PT is pretty disrespectful and it's tiring seeing all of the shade they throw at it. And I don't even think those movies were that great, honestly, but ignoring what was established is just lame. Granted, this particular issue isn't that important in the grand scheme of Star Wars, but just the principle of ignoring established canon just because they don't like it bothers me.
Maybe Vader meant that Luke's ability to construct a saber from zilch with zero training on the subject was a sign of complete training and extraordinary Force awareness. The younglings had the entire Jedi temple to draw on.
Just cause he's called a Jedi Killer, doesn't mean he solo'd an entire Temple. I'm kinda with Ares on this with his fellow Ren Knights helping. Or unless the number of Jedi during the time were really, really small.