It's all but guaranteed as soon as it gets a sequel that it won't count anymore. This argument will be bandied about until Nintendo creates a new video game IP every month, and even some of those will be written off arbitrarily because people making this argument don't care about them.
Honestly, unsubstantiated and unnecessarily generalized claims like that are some of the reasons why my enthusiasm for interacting with people in the gaming enthusiast community has been drastically lowered over these past few months; there are only so many times I can see the same thing said over and over again by people who should honestly know better.
It's correct, on a technicality. It's the officially the first new IP from EAD since Pikmin. Pretty sure they referenced that point in the Splatoon Iwata Asks.
Obviously there have been many new Nintendo IPs from other studios in the meantime.
But, that's not really true either...
Are we (...and Nintendo???) not counting things like Nintendogs (EAD1), Wii Sports (EAD2, Splatoon's team), Wii Fit (EAD5), or Steel Diver (EAD5)?
I mean...three of those new IPs helped propel Nintendo's 7th gen consoles into the stratosphere, it makes no sense to discount them when they were incredibly important to their success last generation while also (arguably) being completely new experiences in the medium.
I guess a more specific claim would be "None of the EADs have produced a new traditional character/lore-driven IP in 15 years" (which I'm not even sure is true, correct me if I'm wrong).
I can honestly understand how some enthusiast and enthusiast Nintendo fans are irked by that even if I don't really care myself.