There's an Ambassador in Altdorf. If it's still Daroir, then they are personally aware of who Mathilde is, both from the dark elf prisoner, and from the Waaagh Lectures. They must also be aware that the Empire is building ties to the Eonir, and since one of Finubar's major policies is to reconnect to Laurelorn and Athel Loren, so he's probably paying a lot of attention to the humans travelling in and out of Laurelorn. At the very least, he might be aware that a number of wizards and a dwarf have travelled to Laurelorn, and would be interested to know what they are doing together.
Additionally, Mathilde hasn't shown much discretion about the waystone project (or at least, by Grey Wizard standards). So far she's spoken about it to other colleges, two dwarf kings, the Tsarvich, and a Bretonnian duke, so that's a very small group of people who directly know about it. However, the Damsels seem to be collectively aware of it, based on that one conversation we had at the wedding, and the Sea Elves of Erengrad have probably heard about how the Witch of Erengrad was sent south on the Tsarvich's orders.
So that's already multiple vectors Ulthuan can learn of the project—from their Ambassador watching the alliance, from their merchants in Erengrad spying on Niedzwenka, or from hearing it from the Damsels, assuming they have some sort of presence in Bretonnia (which I'd be surprised if they didn't).
Oh, and I suppose a lot of eyes from Marienburg are on the Middenland-Eonir alliance as well due to the trade war with the Empire, and we suspect they were able to get a warpstone infused mine into the Skull River, so who knows how good their spy network is.