"Note that because plus is used in a negative context here, you don't pronounce the final "s"."
There are actually rules that specify in which context to use which pronunciation of plus. One such general rule is that, indeed, in a positive context you pronounce the -s and in a negative one you don't. There are a few other rules plus the case of a liaison with the next word (if that starts with a vowel).
Rather than duplicate what's available online, I'll refer you to a page I found helpful (there's also a worksheet to test your knowledge at the end): http://frenchyourway.com.au/how-to-pronounce-plus-in-french
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