Devuan is a fork of Debian, optimised for use without systemd. It has its own repositories which are virtually identical to Debian's, with the exception that some packages have been fixed to work without systemd, whilst a small number of troublesome packages have been blacklisted.

If you've really got a dying need to run systemd, there's nothing stopping you from adding Debian repositories, but I would recommend against it.

Those Devuan images were cut for the stable release so they are quite old now. If you run Maemo Leste, you'll get Devuan, plus a newer kernel which is regularly updated and contains a number of fixes for the N900 that would be missing from the Devuan stable kernel.

I guess what I'm trying to say is that the only reason for running standard Devuan over Maemo Leste would be if you want to run an alternate WM/DE instead of Hildon. You're really tempting me to get off my arse and work on those patches that will fix this.