So I just spent about an hour trying to fix/troubleshoot something that may not ever have worked.
I’ve switched from lxdm to lightdm on all my machines. I saw this section of the Arch wiki and thought “Hey, that would be useful on the laptop!”
After spending no small amount of time trying to discover what provides gdmflexiserver (other than gdm) I took to the forums to ask “Am I mad?” However, in the process of researching my post for all possible explanations I inevitably found the explanation myself. That’s Arch way.
They did a dirty hack on their lightdm package so it would work with XFCE alongside gdm.
This then led me to here, then here and culminated in this wiki edit.
By and large the Arch Linux wiki is amazing but it just goes to show the damage that a bit of misinformation can do to your day. Thanks graysky.