If one wants to learn in the best environment, one usually needs to go to university and _pay_ for the education. To go into a highly competitive industry, get _paid_ (usually quite well) and complain that someone didn't teach you the way you'd like to be taught is some kind of astronomical entitlement that I can't even begin to comprehend.
Beyond that, I disagree with your basic assertion. It's a sign of team maturity to encourage mentor/mentee relationships.
But, if you end up at an org that has their stuff together - if you're coming from nothing maybe you show a high aptitude for programming - it is something that should be an aspect of the experience.
I don't expect to be mentored, but I won't give an employer the time of day if I don't see myself growing in the job, so it's the same effect. Personal/professional growth is table stakes.
You can pick however you want to frame it. It's the same end effect: if I work for you and not as a founder/C suite, mentorship on the job is a baseline requirement when we negotiate the terms.
This poster is someone who took the initiative to get better and therefore showed qualities worthy of reward. They absolutely deserve better than they got.
How can you be so sure? You don't know what the compensation of the GP was, you don't know that they weren't / won't be written a stellar letter of recommendation (or better yet introductions) that will land them directly into coveted senior dev land, you don't know that the experience forged won't make the GP an effective startup founder, etc, etc etc.
You deserved better, and I really hope as you moved further into your career that you found it. Finding great mentors and coaches is truly transformative.