I won't say they never ran OpenWRT, but I've used several generations of Unifi AP's and every one used unifiOS which is based on Vyatta. If they did run OpenWRT, they haven't in well over a decade.
I suggest sshing to one of your Unifi APs and verifying. My APs are running firmware version bz.6.5.28 released 3 months ago. It is based upon OpenWRT 17.01.6 according to the /etc/openwrt_release file on the AP.
Edited: Added newlines for file content. I originally posted this from mobile in a hurry. Copy/pasting from the ssh session on my phone resulted in newlines being lost.