I understand what you're saying, but there's also a huge stigma around being a girl and leading with the fact that you have an SO. Like most things socially for women, it feels like you're damned if you do and damned if you don't when trying to befriend men most of the time.
I've personally tried both methods throughout my life, and have lost every friendship with men I've tried to have because of it. It gets pretty lonely, since the other women in my immediate community aren't really interested in my gaming hobbies. I end up a lot like this girl on the post, with no friends of either gender.
Tbh, it sounds like she did try her best to clarify by mentioning her boyfriend before they actually went to the destination. I could easily see that she perceived her friend asking her to go hiking, so she excitedly agreed. Got home, realized what that would look like, and thought she might bring it up in a way that wouldn't look like the "I have a boyfriend, FYI" trope