Not sure how that is animated, it could be frame by frame. If so, there is absolute flexibility by drawing each frame by hand. It also is a whole freaking lot of work. Spine simplifies animating by allowing you to move and stretch a single drawing. By moving and stretching multiple drawings, you can have a whole lot of animation without having to redraw anything. You can of course swap in different drawings as needed, even so much that you use a different one per frame, but avoiding needing many drawings is kind of the whole point.

I'd say you could do something like the video you posted by using a single drawing, making it a mesh, stretching it as needed to get as much animation from it as possible. Only when you have to would you swap it for a new drawing and repeat the process. Eg, the entire flapping mosquito animation could be done with two drawings for the head and wing. The guy sitting at the bench could be a mesh, but likely you'll need a few versions of him to get all the behaviors.

Spine loads individual images, not spritesheets. You'll need to do image editing outside of Spine.

Spine could animate this using meshes without needing so many drawings. You could do it with 4 images: chest, arms, legs, feet. They are just bent using meshes and scaled. If you wanted some of the details like lighting on the chest or the little line when the arm bends, then you could use more images.