But using it for fingerprinting seems dubious. How many iOS users have never downloaded any extra fonts and all look the same?

The following is from https://coveryourtracks.eff.org/

To determine your system fonts, tracking sites commonly display some text in an HTML tag. Trackers then rapidly change the style for that span, rendering it in hundreds or thousands of known fonts. For each of these fonts, the site determines whether the width of the span has changed from the default width when rendered in that particular font. If it has, the tracker knows that font is installed. How is this used in your fingerprint?

The list of fonts you have installed on your machine is generally consistent and linked to a particular operating system. If you install just one font which is unusual for your particular browser, this can be a highly identifying metric.