You don't learn HP-UX by running OS X. Sure, some commands and utilities are similar (vi, grep, find, all the basic goodies), but you're not going to learn ioscan, lanscan, and other specifics on OS X. So you didn't fully know HP-UX.
And you don't know Unix if all you know is ioscan, lanscan (btw, that's outdated since 11.31, it's nwmgr now). If you hire a guy with 2 years of HP-UX as his only experience over a guy with over 10 years of AIX, I feel sorry for your company. The guy with 10 years AIX would probably be a much safer bet.
These "vendor" utilities change, get deprecated, get replaced, get updated all the time. ioscan required a few man page readings in 11.31 since they added the persistent device special files vs the legacy device special files. Same for the switch from lanscan to nwmgr.
What doesn't change is basics. If you know Unix basics, you can figure out the rest of the particulars between AIX, Solaris, the BSDs, Linux or HP-UX. I learned the Basics on Linux and Solaris at home and when I finally started doing this stuff professionally, I knew what an interface was, I knew about ifconfig, I knew POSIX stuff, I knew about CC, LD and how they relate to GNU's stuff. I knew Shell scripting, I knew about Cron (be it HP's proprietary version of Paul Vixie's), I knew about runlevels and init (which incidently, Solaris 10 made obsolete with services and milestones, so much for all that Solaris 9 experience!)
The hardware/vendor stuff ? I pick up as they throw/change Unix platforms under me. Kernel parameters ? Kernel modules ? So what if one uses modprobe and the other uses kctune. Kernel rebuilds ? Same difference.
Only commands and options change, the basics of how you administer Unix boxes pretty much are the same for any of the flavors.
This is the same for a Genius position. No one asks you to know about defaults write and all the hidden options by heart. But at least remember that user preferences are under ~/Library/ while system wide are under /Library/, know what Bonjour is, how it relates to file shares and printers and other network services, know what target disk mode if not the specifics of using it, know how to boot from CD, from Firewire disk, etc..
Hence if you don't know even the basics (that Terminal thing, what's a logic board ?), you're not an asset, you're a problem. That's why they have a test. If they didn't care and we're going to spoon feed you all the info, there would be no test.