That seems wrong. Having made Flash content in the early 2000's, the drawing tools in the software were all vector-based, and vector-based animation was everyone's go-to since frame-by-frame raster would have resulted in too large an SWF file for the Internet of the time. You could animate bitmap objects also, but they were usually used sparingly, and anyways SVG allows that too.

Wikipedia also identifies SWF as a vector format: https://en.wikipedia.org/wiki/SWF