TBH, I'm not sure what exactly is going on, and there is some confusing information...
...but I'm thinking it's not a typo. For a few reasons. As I mentioned, there have been rumblings about PCIe 4.0 support on Z490 for months now, and on all sorts of websites - including Gigabyte's official page, in addition to Asus'. And, typo or not, if a board manufacturer put something like that on a website these days and it wound up being untrue, there would be a class-action lawsuit in a big hurry. (It's already happened, factually, with CPU manufacturer's marketing...I know, as I was actually class member in one such matter).
The Asus table says "Support the 11th gen processor & PCIe 4.0 for PCIex16_1
" To me, that's far too specific to be a typo; I just don't see they'd accidentally put all that, including the fact that it is only for the single PCIe slot.
That would be a lot of characters (and a lot of detail) for a typo.
But, laying Asus aside, there's Gigaybyte. A 12/31/20 press release indicates Z490 boards with PCIe 4 for GPU and storage
and there's an independent third-party article that, while contending that it "makes no sense", it still confirms the support is there: https://www.pcgamer.com/intel-z490-pcie-4-support/
And I don't necessarily agree that it makes no sense, BTW. My advice previously was to avoid Z490 for anyone looking to upgrade over the past few months, because (at that time) Z590/Rocket Lake was the only assured
way to get PCIe 4.0. However, for anyone who just really wanted to buy into Z490 - for whatever reason - this could be big (if it proves to work). I still believe that then (as now) the way to go is wait for Z590/Rocket Lake (if you're an Intel fan). But again, for people who bought boards long before Rocket Lake was reality...well, this development means they can get PCIe 4.0 functionality without having to replace both their motherboard and CPU
There's another article that ostensibly shows features and boards from the big-name board houses, specifically as relates to the subject of PCIe 4.0: https://wccftech.com/z490-motherboards-pcie-gen-4-support-detailed-asus-msi-asrock-gigabyte/
There's this site which supposedly lists all the PCIe 4.0 boards: https://itigic.com/all-compatible-models-of-z490-boards-with-pcie-4-0/
and there's this one as well https://www.overclock3d.net/news/cpu_mainboard/pcie_4_0_support_on_z490_-_what_the_deal/1
So, there's way too much (in too many different places) for this to be a typo.
Having studied whatever I can find, I believe a likely scenario is that there was pressure from Intel on board makers not to advertise or market PCIe 4.0 on Z490 (possibly even an NDA), because of Comet Lake lack of PCIe 4.0 support. However, it also appears obvious there was some 'behind the scenes' communication and activity, because the board manufacturers went to considerable lengths to incorporate PCIe 4.0 components on their Z490 boards which supposedly weren't even going to have support for it.
The guy in the article above - who said it made no sense - was obviously unaware that the manufacturers knew something that the general public wasn't privileged to. It does
make sense...because now, there's an advantage for any motherboard maker who can say their boards support it.
Granted, there does appear to be some confusion. While I'd prefer to rely on official websites/info from the manufacturers themselves, it's already out there to some extent. And I'd like to see specific tests - but therein is the problem: Until the CPUs are released, there's no way to test it. But it is clear the board houses have known for a while.
With all the above said, if you had already bought into Z490 and this winds up being true - especially if it applies to Rocket Lake's four PCIe 4.0 lanes for a single M.2 storage device (as opposed to 16 for the GPU alone) - then it's an incredibly big development.