There is a difference between:
1. learning outside of work for your own personal enjoyment;
2. learning for professional growth and;
3. for professional needs.
Category 1 is where learning for free, at anytime is valuable, you are doing it as a hobby or as a personal curiosity, so taking your own time for that is fun and enjoyable.
Category 2 is murky, I've definitely done a lot of studying on my own dime to improve myself or learn technologies I could apply on my day-to-day work but... When I could I did it on company's time, on this category I'm using my time for something that will have a direct return to the company, if I can get paid for it I'd always choose so.
Category 3 SHOULD NEVER be done on your own time.
This mentality of needing to use your own time to do any kind of professional development only fosters the race to the bottom, where you are required to use your own time to improve yourself because the company you work for is only interested in extracting maximum value out of your billable hours...
I agree that in the parent's comment case they could have used some of their free time to improve their backend skills and then try a lateral move in their job but it shouldn't make you "amazed" that someone isn't willing to sacrifice their free time to earn a little bit more of salary while their company extracts a higher multiple out of that same work.