That's true as far as I'm aware. In 2019, the DVLA altered the rules governing power and weight figures of the motorcycles allowed for passing tests. This included minimum engine power rising from 40kW to 50 kW. The changes ruled out a fair few bikes that were often used by newer riders to pass their tests. These include the Harley-Davidson Street 750, Honda NC750X, KTM 690 Duke, Suzuki Burgman 650, Triumph America, Triumph Scrambler and a bunch more.

As you know, riders must go through a staggered licencing protest to ride motorbikes, unless they do direct access at 24yo+. The testing is done in this way to ensure those people who are passing tests are as good at riding high-powered motorcycles and scooters as possible. As annoying and nonsensical as it seems, it's essentially been done retroactively to keep riders safe (especially those that are more young and inexperienced). Unfortunately, laws and rules sometimes change after the fact for various reasons. To ride the NC750X on an A2 licence, I'd suggest getting it restricted slightly.