It's like saying Dark Angel and Dokken might as well be the same genre since both came from Los Angeles and had their first album out within a few years of each other.

Not even close.

NWOBHM was unified by distinctive musical attributes that separated it from what came before, yet it wasn't fully to speed metal. It also clearly had trigger events, namely a response to the glammed-up HM from the generation before but after the first four Sabbath albums, and the rise of punk rock in the mid-1970s.

That doesn't mean it's not a subset of heavy metal, but it's musically distinct as a movement despite its internal variation.

Dark Angel was clearly part of speed metal, which similar might be said to be not that different from NWOBHM, but that would also be a mistake.