>Go build it. See how many people discover yourGitHub repository without so much as a coherent explanation of what problem your code solves. (And, yes, that's marketing.)
Only in a pedantic sense. We could live with non-commercial marketing, if you want to call it that, without an advertising budget, ads, sponsored articles, and other such BS - like, say, the author promoting their FOSS on their blog. Or crafting a nice looking page with an enticing description.
As for commercial companies not being able to sell their products in that case, the world's smallest violin plays a heart-felt tune for them...
It's by no means pedantic. A lot of companies don't do a lot of advertising or sponsored articles but do spend a huge amount of money on informational articles (i.e. content marketing), trade show booths and paying employees who have talks accepted there to attend, developer days, webinars, meetups, other forms of engagement with developers and other users, etc. (And coherent copy on the website which sure doesn't come for free--which is also marketing.)
If I had a nickel for every website I opened up and then closed because after 5 minutes I had no idea what the company or product did or why I should care, I'd be rich. Might be something really clever. Dunno. I just know it's 5 minutes of my life I'm not getting back because some techie thought their genius would be obvious to all.