This comes down to how strong your claim is, and what point you're trying to make.
If you're saying that compiling codecs in Fil-C is not a good use of Fil-C in general, then that's wrong, because not all codecs have a WUFFS version.
If you're saying that compiling WebP with Fil-C is not a good use of Fil-C because there's a WUFFS version, then that's also wrong, because I can't use the WUFFS version in a fully memory-safe web browser, since WUFFS doesn't support Fil-ABI and nobody has written a browser where the whole transitive closure of everything that the browser uses is WUFFS or some other safe language.
On the other hand, that is possible with Fil-C. (I'm posting this comment using a memory safe browser built with Fil-C, which has WebP built with Fil-C as well as everything else down to the libc/libc++ layer built with Fil-C.)
> double-free
Double-frees are deterministic panics in Fil-C. So yeah, Fil-C catches that, and any other memroy safety violation that could be used to achieve weird execution