This can't possibly be true, since the Linux kernel's code is GPL and approximately every tech company uses Linux.

Note that this is about source code, not binaries, or nobody would be working with docker (and more.)

Of course a company must forbid copy/paste of GPL code, because that would GPL the codebase and that's hardly what they want. But one should ask the Legal office (and/or other offices) about adding any MIT, BSD or proprietary library: credit must be given (how?), licenses must be available and compatible with the way the software is distributed. There are so many licenses out there, everything should be vetted.

I'm self-employed in Germany, and when I was employed, what was in the contract went more in the other direction: it was explicitly allowed to contribute to FOSS projects. Of course, it still would not have been OK to randomly add GPL software to a closed source customer project. If anyone had been stupid (uneducated, really) enough, somebody else on the same project would have noticed.

I was at a FANG until 2022, and we were allowed to introduce GPL code into the monorepo. There were policies on how to do it correctly, but definitely not prohibited.

(AGPL, however, was nearly impossible to get permission to use)

Companies not wanting GPL code in code bases they want to keep proprietary should not be surprising to anyone. I fail to see how that is relevant to the comment you are replying though.

Some companies subscribe to FUD (aka lawyers covering their ass) and forbid use of AGPL, GPL and sometimes even LGPL software outright even though they allow proprietary sofware that has even more restrictions, but the big elephant in the room that is usually cited for these open source to "proprietary but we still want the publicity of open source" license changes (AWS) is not one of those companies that put fear over profit.