Yeah, since the M777 doesn't have a napalm shell [1] that would be totally absurd! :D

In fact, since I got curious, it seems nobody fires napalm out of howitzers, it's non-trivial to combine the thin easy-to-break shell skin needed by napalm with something tough enough to be fired from artillery [2].

[1]: https://en.wikipedia.org/wiki/M777_howitzer#Ammunition

[2]: https://www.quora.com/Was-napalm-ever-used-in-artillery-roun...