Yes, OpenFl reimplements the Flash API. If you want to target something other than flash, you can still write against the flash API with -lib openfl and then the flash types are remapped onto the OpenFL implementation: openfl/lib/flash-externs/src/flash/display/Sprite.hx at develop · openfl/openfl · GitHub
That or you write against the OpenFL API and when you compile to flash, it uses the flash API directly: openfl/src/openfl/display/Sprite.hx at develop · openfl/openfl · GitHub
FWIW there’s also a more lightweight fork that is for the browser only: GitHub - innogames/openfl: The "Open Flash Library" for fast 2D development … it doesn’t have all this remapping, but the Haxe compiler specifically has the --remap argument if you want to map flash API to openfl API or the converse.
Best,
Juraj