IIRC it was even weirder, because it was a XP branded Server 2003 under the hood.
It's even weirder than that :)
The initial release of XP for Itanium was more or less a stripped down XP build with limited 64 bit support. They then quickly gave up on that approach and released a second version of Windows XP for Itanium, together with the launch of Itanium 2 support, which was, as you said, a rebranded Server 2003.
It was still exciting at the time.