I came up with a solution. If I go to the "Security & Privacy" settings and go to the Locking tab and set "Lock on suspend" and "Lock after screen turns off" both to OFF, I can wake my laptop back up without having to open the laptop lid.
Perhaps related to these:
If you also add to the issue on github maybe it will get more attention! :)
This issue is more on lightdm, I've already explained this before so I won't explain further and rather I'll just put steps here, This problem is because the power settings are used from the gconf configuration of the user lightdm, and not the current user. Just test:
su - -s /bin/bash lightdm
gsettings get org.gnome.settings-daemon.plugins.power ...
For what it's worth I think this behavior may be related to Ubuntu 18.04.
Looks like the same problem was reported here
For what it's worth, your system isn't actually sleeping, it's suspending. I'd love to be told otherwise but it seems to me like Elementary doesn't actually properly support sleep but instead suspends your laptop.