Completely detached from the more serious matters described in the article, I thought I had read that we should refer to they/them the same was we refer to "you

Completely detached from the more serious matters described in the article, I thought I had read that we should refer to they/them the same was we refer to "you". But this is the first time I've noticed this formulation: >[...] whose pronouns are they/them, [...] Simonis, who is also [...] I guess my brain had trained me to expect "who are" rather than "who is". But apparently "they is" is acceptable? Or was this a mistake in the article?