From what I understand, it was attempted to use lightdm for the lock screen as well as the login screen, but it was scrapped and pushed to implement in a later release because of various bugs and issues with nvidia drivers or some such. Also I read that the reasoning was in favor of the LTS release, perfectly understandable.
I saw that at one point it was implemented in a beta, so it seems possible. I have an intel onboard gfx card and I'm not sure whether or not I would encounter issues trying to make it happen, but I would like to give it a shot!
I have no idea how to go about it, could someone please provide assistance? Thanks!