I'm not sure where people get the idea that they can assert something as a counterpoint on no basis. "That's just your opinion," is not a rational basis to disagree, especially when your counterpoint requires the implied basis (semantic) to be equally relevant to the original argument's basis (law).

"Open source" isn't based on the semantic that the source code is public, its definition comes exclusively from a set of guidelines that limit the rights of the original creator to enforce a specific range of IP protections guaranteed to them by domestic and international laws. There are various types of public licenses, also known as GNU, that are considered "true open source" in that the original creator can only enforce some provisions like a source attribution credit in a modified distribution. And there are permission licenses, like the MIT License, which don't necessarily meet every guideline for FOSS, but do meet most of them.

When it comes to IP, the only basis that matters is legal, as its the only basis that's universally enforceable. Laypersons can define "open source" however they please, but when engineers, devs, business, and legal discuss it, it has one definition, and that definition is well-established as exemptions from current IP law.