Regardless of the conscious decisions of Wikimedia designers, it seems
that such decisions are overridden by the conscious decisions of OS
and browser vendors. As long as web fonts are out of the table, it
seems that the difference between "sans-serif" and a full fledged font
stack including proprietary fonts is none for the majority of users
(Windows and Android), slight for the minority of Linux users, and
then, yes, Apple users get Helvetica Neue instead of Helvetica.