Renaming an extension does not actually modify the contents of the file. Some software that supports webp may look at the contents and recognize it as webp and load it. But that’s not all software.
It’s a bad practice to do that. It’s like changing a .docx to .pdf and expecting it to be fine.
It's absolutely not a great thing to do in general, however it does work since the file contents are apparently a modified png. And everything that I used to both look and modify it later including automated CLI level cropping commands seem to think it's a png.
Since it works and hasn't broken anything, and nothing I have recognizes the webp extension, seems to be okay for this niche purpose.