I think it's just wrong. Related, linked recently here: https://dwest.web.illinois.edu/grammar.html https://dwest.web.illinois.edu/grammar.html The authors are

I think it's just wrong. Related, linked recently here: https://dwest.web.illinois.edu/grammar.html https://dwest.web.illinois.edu/grammar.html The authors are biologists, so I suspect they're not particularly versed in mathematical writing (and that McCarthy was not likely providing them much advice on it).