The difference is that all existing Sketch files are guaranteed to match the file format specification that has been published, whereas Figma could change their file format entirely without telling anyone and suddenly nothing but Figma can read files exported from it until third party devs reverse engineer the new format (which isn’t a given).

Figma would earn a lot of goodwill if they published their specification and assured users and devs in its ecosystem that the format won’t change out from under them. Even then though, they could very easily say one thing and then do another. It’s an unavoidable uncertainty that comes with web apps.