> Wasn’t Windows back then dependent on vendor supplied device specific display drivers for anything above basic VGA?

In theory, yes, but the reality was starkly different as I recall (I worked there in the late 90s). In my experience, Microsoft was doing a ton of heavy lifting helping vendors with device drivers.

I remember using a 800x600 16 color generic SVGA driver with early Windows (3.11 or 95?). At registry level, I guess the mode was made mostly standard as a derivate of the 640x480 16 color standard VGA mode. Indeed both modes fit within the 64 kiB segment using 4 layers and matching the 256 kiB of VGA memory.