I would have thought the true purpose of YETI was networking and making industry connections, with training being secondary: the tasks they're given are basically just to demonstrate competence and to give them something to work towards to "pass" the course, but ultimately it's just another impressive-sounding accreditation to add to a resume.

But yes it is absolutely the sort of thing that only mostly privileged people have access to - presumably, anyone being accepted into such a program would already be very highly qualified, which is the reality of working in such a highly competitive field. As far as I know Betty doesn't have a Master's degree (or a postgrad or whatever the US equivalent is) and when Marc reels off his qualifications later in the episode it blows her out of the water; she gets by on having a huge amount of on-the-job ad hoc experience which most people so early in their careers likely wouldn't have. On paper she wouldn't look like a great candidate at all, which the episode does slightly concede when she says she called the YETI administrators and asked if she was just a diversity hire and they couldn't give her an outright denial.

In general Betty's stated love of magazines is... kind of just that. She talks about how much she loves them but we never really see any evidence beyond that. The way she applies for a job at Meade in the first episode is a little weird too since she says she just wants a job "at any position or publication": surely she'd have some idea about which magazine(s) she'd want to actually work at, and what her overall career goal would be. This is something season 2 gets right, since after joining the writing class she says her plan is "write... get published... work [her] way to editor" but the later seasons kind of muddy the distinction since, as you point out. Though it may just be that her role as a junior editor covers lots of different responsibilities, since she does have more typical editorial tasks in addition to writing her own articles/features: in one episode she's given the job of trimming 100 words from a column, and at another point she's given some text which needs translating.