> a language defines its own conventions

Where are these mythical languages? I think the word you're looking for is syntax, which is entirely different. Conventions are how code is structured and expected to be read. Very few languages actually enforce or even suggest conventions, hence the many style guides. It's a standout feature of Go to have a format style, and people still don't agree with it.

And it's kinda moot when you can always override conventions. It's more accurate to say a team decides on the conventions of a language.

> No, they're absolutely correct that it's critical in professional and open source environments. Code is written once but read hundreds or thousands of times

What you're saying is reasonable, but that's not what they said at all. They said there's one way to write cleanly and that's "Standard conventions", whatever that means. Yes, conventions so standard that I've read 10 conflicting books on what they are.

There is no agreed upon definition of "readable code". A team can have a style guide, which is great to follow, but that is just formalizing the personal preference of the people working on a project. It's not anymore divine than the opinion of a "rando."