It works because the .png and .jpg extensions are associated on your system with programs that, by coincidence, are also able to handle webp images and that check the binary content of the file to figure out what format they are when they're handling them.

If there's a program associated with .png on a system that doesn't know how to handle webp, or that trusts the file extension when deciding how to decode the contents of the file, it will fail on these renamed files. This isn't a reliable way to "fix" these sorts of things.