They've said for several years they were debuting a new bg and had auditions for it ("bring the boys out") so that's never stopped being on the cards, I think. They just kept wanting to milk NCT because it was so lucrative for them. Knowing that albums and merch are the top two revenue generators (per their own sales reports), they've been able to sell millions of copies of NCT 127, NCT Dream and NCT each year for about 3 years now. Even a lazy ass release like universe 2021 still sold a crapton. (NCT U is technically a sub-unit but they mostly just use them for the combinations on the all-unit NCT album).
Dream and 127 are selling out domes and the bulk of the SMTOWN show sales came from Czennies. NCT Japan will be a strong debut since Dream and 127 are already both very popular in Japan (and don't seem to have cannibalized each others' sales like I thought they might.)
I've said if anyone in NCT has to worry, it's probably WayV but they've shown strong on this last album and the fanmeets and represents markets with opportunity if they pivot them slightly.
1 NCT sub-unit repackage can sell more than SuJu+Shinee+TVXQ combined and the NCT-Sanrio partnership shows the merch $ of having so many members, so I don't think anyone is going to walk away from that $$. With another all-unit album planned, that's even more $$ because sub-unit albums always have a sales spike after.
To answer your question,
@Abeamus, NCT is a little of both. The company considers it one brand with multiple sub-units, but fans look at it two ways: 1. Multiple groups under 1 group, like the company does, or 2. Intensely focused on their sub-unit and considering the other sub-units and even the brand as separate (this is usually NCT Dream or WayV fans, who sometimes don't consider themselves part of NCT.) Because they mostly focus as autonomous groups and have since the start, it's easy to separate it in a way that you wouldn't do EXO-CBX, for example.
The bg list is also missing the sub-units for the other groups: SuJu and EXO both have sub-units and there was talk of reactivating some of them, like EXO-SC. And then to make things even more complicated, the NCT sub-units have sub-units too.

(They first tested them in WayV as part of the Lucas problem, and it now sounds like they're rolling out for 127 too. They should use the freaking NCT U unit but whatever.)
I definitely agree with everyone that it's a mess and I don't understand how they intend to debut multiple new groups with such a large roster and NCT solos on the way too1