Defence is not the strong point of this or any Ajax in history. But tactics aren't limited to defending and it's not that easy to make this Ajax defend. The Ajax that beat United at OT was very young too, and mainly more defensive because it wasn't very competent in attack. Clubs like Schalke and Lyon can spend on players 5 or 6 times more than Ajax, these players are generally better and more used to a high pace game where players run faster, turn quicker, jump higher etc, it would be really embarrassing if they couldn't pose any threat to Ajax at all.
But Ajax plays a much more sophisticated game in which all the players move and pass to an attacking plan. They can move the ball around very quickly because they are selected on and trained in positional awareness. You say tactics were out of the window, Ajax supporters watching this United would say that only very basic primitive tactics made it into the building. That's something any team has to find an answer to, no matter how much better the players are individually. To be honest, I don't think Pogba would have made it into this Ajax, not after he got properly schooled and trained in vision and awareness. It's not just a difference in level, at Ajax there's more tolerance for defensive mistakes, but little tolerance for indecisive dwelling on the ball and lack of awareness about what's happening behind a players back for example. All the players need to have a properly developped and schooled football brain to play these attacking tactics. Tactically Ajax is well ahead of any PL club, that's why they are in the semi's in the first place.