> This smells like a small-minded startup-bubble nonsense to me

Small-minded, no doubt (though "business"-minded rather than specifically startup).

In my national context (Australia), the vast majority of companies, small and large, operate purely in the fundamentalist capitalist mindset. There is no 'should' (conceived ethically) in their stock of concepts. It literally doesn't exist (where it occurs it's just a PR exercise, which the PR department will translate into management's native dog-eat-dog language for their comprehension in meetings).

Ideally we wouldn't work for such companies. But the stock of jobs at better places is limited, so by statistical definition 'most' of us won't be there.

Lots of companies use open source code but don't actually require any changes to be made. A lot of industry standard open source software requires no code modification of the source for the vast majority of applications. Examples being Nginx, Python, Apache Web Server, Tomcat, pretty much any enterprise level application. So many people use it that for the 99%, the code already exists, with 99% of the use cases being heavily tested.

Now they could be donating some amount of money, but that's in no way required.

All good. But then, the same people could contribute code non-pertinent to their product as open source. As someone else said in a comment here, you can always find what to open source. There is really no excuse.