As far as I'm aware, you can't just export a geometry/point cache or vertex animation to Sketchfab. It does support Blend Shapes and Joints though. So you'll have to somehow bake the deformations into these concepts. Unsure how to do that in Modo I'm afraid.
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…