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What does the author have a against a text size that’s readable?

The page does not set a text size. You are complaining about your own browser's defaults.

Browser defaults suck, then.

This is classic "works fine on my machine".

The default for paragraph layout does suck indeed. But at least the font you should be able to customize.

It's larger than HN on my browser. It seems to just be what you have your browser default to.