Eh. It's been pretty easy to fingerprint browsers for a while now, including those types of CSS hacks. The real feat is doing so without looking like you're doing it and for it to be durable (survive OS upgrades, reboots, etc).
navigator.useragent will still have more precise answer as the amount of people who fake their useragent (or have JS disabled) are less than the amount of Windows/Linux users that have Helvetica (clone/real one) installed.
https://fingerprintjs.com/blog/disabling-javascript-wont-sto...
basically they use CSS trickery together with server-side stuff.
It's pretty clever.
to detect font (which detects OS), and to detect browser features (which detects browser)