Specifically Dream 
not even nctzens claim that
I feel like those that group Dream as 4th gen do so because the group's age was quite young for 3rd gen (1999-2002) but more on par with other early 4th gen groups. Plus they seemed to have their biggest hits during 4th gen. But debuting in 2016 makes them 3rd gen. The only NCT unit that is 4th gen is Wayv (debuted n 2019). And I'm not sure if Wish is 4th or 5th gen.
Eh. The idea of generations in K-pop as it's currently used doesn't make much sense.
I feel like generations on make sense when talked about things in a series or lineage. For example, After School's graduation system is something where it makes sense since you assign generations within the continuous group "After School" either to income members or iterations of the group.
Another way would be to describe groups coming from a company since the company is debuting the idols, so it's debuting them in a series. For SME, for example, you can assign them to gendered groups:
1st gen: SES
2nd gen: MILK
3rd gen: Sugar
4th gen: Isak 'n' Jiyeon
6th gen: SNSD
7th gen: f(x)
8th gen: Red Velvet
9th gen: aespa
10th gen: Hearts2Hearts
Since it's companies that are forming groups in contrast with previously debuted groups and within the context of the contemporary scene. So Sugar wouldn't be a first gen group but would be SME's third gen girl group.
In order to compare groups, we could always focus more on the type of music they release and compare that way rather than arbitrary dates. We could talk about the changes in girlcrush throughout the years or other characteristics within a period of time. Much more flexible imo.
So if they want NCT to be fourth gen, then I don't see why not. You can probably rig up a justification no less strange than curent gen talking points.
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