Setting the license text is an explicit act and it seems fairly unlikely for anyone who creates software to think they can relicence GPL code or to think they didn't need to Google it first. Doing something that you meant to do isn't a mistake it's a choice.
It seems more likely that they didn't think anyone would notice.
> It seems more likely that they didn't think anyone would notice.
Maybe, but if that's what they thought (and I have no idea, I haven't spoken to them or anyone else about it), it's very foolish, because this kind of thing will always get noticed eventually, especially if the project becomes successful.
YC tells founders that one of the fastest ways to kill your company is to base your product on code that's not legitimate to use (i.e., that you didn't write yourself or that is used in breach of its license). That's because it's one of the fastest ways to kill funding rounds, acquisitions and enterprise deals. Not everyone listens or understands.
It even asks (or at least it did the last time I checked) in the application form, if you wrote your code yourself, to raise the issue of IP ownership/licensing from the start.
It seems more likely that they didn't think anyone would notice.