According to their pricing page: "We provide free Gold and Ultimate licenses to qualifying open source projects and educational institutions. Find out more by visiting our GitLab for Open Source and GitLab for Education program pages."

I'm sure Wikimedia has the dosh to pay for licenses themselves, but it's hard to see how the per/user pricing model would work for any open source project.

No, Wikimedia is going to be using the Community Edition (CE) of GitLab, which is free and open source under an MIT license. This decision and the reasons for it are described in more detail in the FAQ section of the linked article.