The reality is that spending your free time on job-related stuff is likely correlated with improved job-related skills, though. It's not a correlation of 1 obviously, but it's probably not 0 either, so it is relevant.
Except that studies show reduced capacity to work after a certain number of hours. Does doing the equivalent activity to "work" at home count against those hours? I don't know the answer there.
We do know that diversity increases the effectiveness and problem solving ability of a team - at worst, getting a bunch of people that have home labs is actively detrimental to this, and at best does nothing to improve it.
There's limited time available in interviews, and you're probably better suited asking relevant questions.
(And from a personal perspective, asking about home lab details would benefit me. I've got ten gig fiber throughout the house and close to a terabyte of RAM consumed by my VMs... But none of that is realistically making me a better employee)
Interesting, I'm not sure if it counts against one's ability to work, but it seems like it might, since it's a similar activity. And you're right that other activities can also yield helpful skills.
I agree that competency testing, especially project based, is better. The only reason I'd ask about their off time is to get a sense of what they like and better getting to know what kinds of projects they might gravitate to, given the choice.
