That's a bit of an exaggeration. Mozilla's position statement on web packaging[1] says:
> As a whole, and for origin substitution in particular, until more information is available on the effect on the web ecosystem, Mozilla concludes that it would not be good for the web to deploy web packaging.
Personally I'd like to see web packaging adopted without the origin substitution capability (at least to start with), as it would still allow sites to offer web apps comprised of a fixed bundle of code (signed with an offline key), rather than potentially different code each time you visited. That would reduce some of the concerns around serving "secure" apps on the web, as long as browsers had a way of preventing the server from silently updating the web package (perhaps using short-lived unique subdomains/certs).
[1] https://docs.google.com/document/d/1ha00dSGKmjoEh2mRiG8FIA5s...
[1] https://webmasters.googleblog.com/2019/04/instant-loading-am...