Yea, "Desktop Boards" refers to Intel's Desktop Board products; it is not for boards from other vendors.
If your board is from AsRock, then you should be getting your drivers from AsRock. They are the only entity that will do interop testing of all drivers necessary for the components used on your board (from other vendors as well as from Intel). By "interop", I mean verifying that the driver packages and drivers work well together. If AsRock is not posting updated drivers, you should be complaining to them. If they don't respond, well, pick a different motherboard vendor next time (if everyone did this, they will eventually get the message and provide better support).
Since Intel never released a X99 board of their own, there is not going to be any one place that you can go to to get a set of drivers (well, other than the AsRock site). As JFFulcrum pointed out, you can search for and download Intel LAN and Chipset drivers directly. Indirectly, you can also locate updated ME and RST drivers through the download pages for the NUC and 8 Series desktop board products (though these may not be optimal for your particular board). You will have to hit Qualcomm's site for the drivers for the second LAN port and Realtek's site for the driver for their audio codec.
Ah, I did not know that!
Thanks so much for the simple and straightforward answers JFFulcrum and N.Scott.Pearson.
I'd asked this question on several forums and was finding it surprisingly hard to get an answer!
So is the Intel Driver Update Utility similarly designed for Intel-branded boards? That would explain why it NEVER finds updates!