Yeah that's what I expected then, also this is on their animation documentation: https://help.sketchfab.com/hc/en-us/articles/203058018-Animations So yeah, you can only animate a whole object using transforms, you can't upload a vertex animation. You will have to find some way to bake it to bones, or blend shapes. Below…
I'm no blender user, but looking at the documentation, shape keys seem to be blend shapes/morph targets. Whatever you want to call them, the concept is the same. So if that script/tool doesn't work for you, you can try follow my steps above.