That sentence was the only part of the argument using the idea of something being "reasonable".

Whether you agree or disagree with their general stance, the word "reasonable" isn't load bearing.

In particular they didn't say that limiting cell phones was reasonable, or requires parents to be reasonable. They just wanted an example of parents enforcing boundaries.

The only load-bearing part of that sentence is the idea that parents do enforce those boundaries. Which they do. It's irrelevant if they are doing it because they're "reasonable".

TL;DR: I do know what the discussion is about. But your comment wasn't about the general discussion, it was about the quality of a specific point, and I'm defending that specific point.