I either ran into a bug attempting to use variable lux in Room Lighting or am very confused, but there's nothing in the docs about the feature.
I tried creating a couple hub variables and that didn't populate the list in RL. Then, I was looking back over my research and saw mention of a connector so I went back to my hub variables and chose to create an illuminance connector for one of the 2 hub variables. Then I went back to the Room Lighting instance and now BOTH of the hub variable are there in the drop-down.
variables:
RL:
OK...even more strange. I just edited one RL instance to make use of one of the variables (the one I didn't create a connector for) then moved on to the other RL instance I wanted to edit and I'm back to an empty list for that instance of RL:
So currently one RL instance is populating with both and another has none.
There is some code around when it fetches the list of Hub Variables, and it doesn't do it frequently enough. There's been an issue with this, that should be fixed in the next release. In the meantime, hit the Update button in the RL instance, and it will update the list of available variables.
In latest release it loads the list of variables whenever you open the app. But, it doesn't do that the very first time if the RL instance pre-existed the last release. Hitting Done or Update will put it into the mode where it fetches the list every time it is opened (or Update hit).
To add to the above, you do not need a connector (for this or almost anything, but certainly not anywhere that says it lets you use a "variable" directly), so this was just coincidence. I'm not sure where you read about them, but if it was in the docs and you think there is something there that hints that you might need one, feel free to mention where so it can be clarified.
