They're clearly a bit over-zealous bout what examples they think have meaning. They cite substitution as a good test for a phrase but double down on boiling water.

> Lexicographers used a substitutability test: if you can swap synonyms freely, it’s not a lexical unit. “Cold feet” (meaning fear) can’t become “frigid feet”—so it gets an entry. But the test cuts both ways. You can say “boiling water” but not “seething water” or “raging water.” The phrase resists substitution too.

These aren't failures for substitution because "Raging" isn't' a synonym in this case. where frigid would be a reasonable.

I wonder perhaps if the author is confusing the idiom "hot water" which is in there https://en.wiktionary.org/wiki/hot_water and would fail the substitution test.