I believe they mean that vendors support the FOSS since it’s economically advantageous for them to do so (usually bc implementing an alternative is not economically viable). The proprietary part finances their contributions to Foss, which is usually the platform that they run on top of.
There is a more detailed explanation on the economics of Foss here, for instance: https://hachyderm.io/@anthrocypher/112315622785685958, but as I understand it it’s a common good that companies try to build on top of (and in some/most cases supplant with their own proprietary versions).
But yeah, I’d love to see the OP’s thoughts on this.
I understand you may feel conflicted about your prospects about the current job market and the conditions of the industry in general, but it isn’t all bad.
Like other people said here, before throwing the towel try some things. Some domains may be “better” for your end goal than others - say, if you’d like to contribute to the Linux kernel, maybe you should try getting a job that is “closer to the metal”, maybe working on performance in some PaaS?
Different companies and industries have way different cultures, some that may be better aligned with your personality. In my personal experience, I always felt like I had more autonomy both over the software and the projects in smaller companies. Heard some people mention a similar sense of cultural fit in bigger, “hard” tech-focused companies (e. G. Embedded software), so once again, I recommend you experiment.
You most likely will benefit from the experience, and it may be a good stepping stone towards your end goal. Your day job does not define what you do and who you are, so you don’t need to assume that it’s 100% Foss or nothing, there are many possible paths in between, like other people have suggested here.