As someone who struggles to find enough time for side projects, I agree that it isn't necessarily a sign that someone isn't passionate about the job. However, if someone says stuff like "I'd never work on something I'm not being paid for, I'm a serious professional that's irresponsible", its pretty obvious that they don't actually like what they are doing.
its pretty obvious that they don't actually like what they are doing.
My dad has been a mechanical engineer for almost 40 years and I struggle to think of anyone who enjoys their job as much as he does. Yet I have never once seen him do any personal "side projects" that are in any way related to his day job.
Many people truly believe in not doing anything related to their career/job outside of their workplace. They do something totally different instead. One might train for a marathon in her spare time, instead of learning the newest JS framework for example.
The side project concept is fairly unique to digital workers, especially software engineers simple because we can. Noone asks an accountant or a secretary for side projects.
pretty obvious that they don't actually like what they are doing
Not to get into a debate, but there are tons of people who don't like what they are doing. But they are still good at it. Sure they may not be truly happy, but they are good at what they do even if they don't like it.