Could be wrong here as ive taken a quick 5 min glance, the frontend (HTML / JS) looks like it wont care if you cluster the backend. The backend (the
go part) may work by default (but it doesn’t look intended to).
It looks like it connects over the Unix socket  to the host (so it assumes LXD running on the same server as the demo server), if you keep creating instances on a clustered host, LXD used to, distribute them across members of the cluster which means technically “yes you can”.
At a guess it will fail when the terminals start because LXD wont re-route the request to the right LXD server with the new instance (thats a guess it might work been a while since ive used clusters, which means this does work).
You are probably far better of writing something yourself, a clustered demo server seems extreme.