It seems the reason they're inlined in the page at all is to measure things briefly like bounding boxes (not sure the full extent as it didn't cover that), before subsequent removal. I'm not familiar with Scratch and its use of user-submitted SVGs but I'd be curious to read more about what they're doing that required it be inlined specifically.

(This isn't a comment on the challenges in proper sanitization fwiw, as I've needed to do various of the same things myself)