Pronunciation of Plus

I'm reading https://www.lawlessfrench.com/grammar/plus/, and there's either an error or an ambiguity.  In the sections of plus used as a comparative or superlative adverb, it mentions liaising, but in the section for its use as a negative adverb, it says "In negative constructions, plus is always pronounced [ply]", with no mention of liaisons, which to me means that you should never liaise in the negative.  It gives the example of "Il n’est plus en France", which based on that rule, would mean you neither pronounce the S nor liaise it to "en".  I asked about that example on reddit, and several native French speakers said that liaising was optional, with some saying it depends whether formal or informal.  So, which is it?