I don't think they forgot. This news item wasn't about "capability" but official certification; I don't think any of the boards you mention have that.
Not that I'm disagreeing or anything, but isn't certification mandatory before a company can even allow a mobo to use Thunderbolt? The story mentions that certification by Intel is still required for now.
If that's the case, the X570 Aqua has had the Intel Driver for its TB port available since its release, implying AsRock already paid for the Certification for the Aqua motherboards (and undoubtedly part of its high cost). Same would apply for the X570 ITX/TB3, assuming it also already has a Thunderbolt driver available.
On the other hand, the Designare was not permitted to allow use of a TB Card despite having a TB header. And either they didn't want to pay for Certification or just haven't bothered to do it, since their X399 Designare support doesn't have any kind of Intel Thunderbolt Driver available. The only person who got TB to work on the Threadripper Designare was L1Techs, and he couldn't share how he did it since it used proprietary code.