There is no editor. As IncusOS has no local shell or SSH support, all interactions with it must be done over the API, so in this case, you’d use incus remote add on your host to add the IncusOS server, then use it as a remote, which will then use your local text editor.
Using incus exec against IncusOS is only there for development/debugging and isn’t part of normal use of IncusOS (and will be removed soon).