This opinion you've found is wholly ignorant of what open source is and how businesses use it.

A great deal of open source software is written by paid developers at large organizations. Oracle, for instance, maintains and improves MySQL which is mostly under the GPL open source license. These developers are employed by Oracle, but the source is freely available. Companies like Google and Facebook also pay their developers to improve open source software they use in their businesses. Other companies that make open source software make money by selling services and support. The software is free to use, but if you want expert help or a specific bug fixed you pay them, and they employ developers.

Edit in response to your edit: Even if the code wasn't on Github an AI could scrape it and learn from it. But LLMs aren't going to replace developers any time soon. They can (and have) reduced toil but they're not going to be able to write applications or deal with vague, contradictory requirements like a human can.