I’m using an Ubuntu VM as a CUPS/smb print server to print to my 2006 HP3600n; I just tried it from my iPhone - worked fine. I see no reason to think it wouldn’t from a Pi.
(I wasn’t even trying to get AirPrint to work; it just worked out of the box and today’s the first day I ever tried it.)
It might be your router blocking wifi-to-wifi lan connectivity, check your settings. My Brother wifi label printer works great with airprint from my iPhone.
(I wasn’t even trying to get AirPrint to work; it just worked out of the box and today’s the first day I ever tried it.)