According to the above article that I linked, the latest HP driver for the Deskjet 990cxi would have been included with the Tiger 10.4.4 that came installed on your iMac Intel:
"All included printer drivers are installed by default with Mac OS X 10.4.4"
I have the HP Deskjet 6840 and it is on the same list using that same 3.3 driver. When I open Applications>Utilities>Printer Setup Utility and then select (highlight) my printer, wait a few seconds and the "Show Info" button above becomes bold so that I can click on it, the report shows Driver Version: 3.3
I did not install 3.3, it was just there in the OS when I unpacked and started up the iMac.
Also, this list from the HP website indicates that the latest 990cxi driver was built into the Tiger software. I did the same search as you at the HP website and see the same thing you found, that it states that the driver is 2.7.1, however, in looking at the date, it is a 2005 date, so that is old, pre-iMac Intel information that I'm sure HP will someday get around to updating. As an enormous company, selling just tons of quality printers, HP has to closely cooperate with Apple to see that Mac users have available, through Apple, the latest updated drivers and software at all times.
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c00317516