As far as Blizzard was aware, they were doing it first. Their alpha came onto the scene before War Wind was released. Their alpha may have predated War Wind's alpha -- can you be sure that's not the case? In that case, they may have genuinely been undeniably 100% the first to do it.
Having said all that, it doesn't even matter. Let's say they were second. Let's say that War Wind's production was so highly publicized (lol...) that Blizzard was aware of what someone out there was doing with a WC2-like engine. They could not anticipate the (barely) positive reviews War Wind would go on to receive. As far as they were concerned, different races and different units was a risk; it was untested, unproven, and liable to blow up in their faces.
That it may not have been THE first on the scene (you've yet to demonstrate this by matching up the Alpha dates) does not change that Blizzard was treading dangerous waters of their own free will. With SC2 they elected to bypass the challenging route. They won't have been the first, second, or third to do anything remotely progressive with the multiplayer gameplay.