> The fact that I do programming at the weekend for fun on my personal projects does not in the slightest imply that I have fun at the kind of project that I have to do for the employer. So, judging by this criterion is a really bad idea of the (potential) employer.

I am not sure that was the argument being made. I think it was more like "If a candidate actively uses and develops their skill set in their own time, then they will be preferred over candidates that don't". Those candidates demonstrate a higher level of mastery of the skills needed by virtue of doing something, _anything_ over other candidates who don't, even if the demonstration is trivial. It can also signal a lower managerial load if the candidate has demonstrated the skill set to self-start and self-motivate. That can also mean less ramp-up time to being productive.