Yes, it is possible they didn't retain knowledge of Arcane Khazalid. I think that's unlikely because a lot of people who knew the language would have lived in the Old World and been unaffected by the Sundering. Because you know, that's where you can collaborate with Dwarfs the best. The War of the Beard would have fucked a lot of them over, but the Dwarf's increasing extermination efforts would have incentivized many to share knowledge about Runesmithing to better contribute to the fighting against deployments of Runecraft.
Those who knew a lot about runesmithing ended up getting evacuated to Ulthuan. It is possible, but unlikely.
You should note that I have expressed a lot of uncertainty about whether it is confirmed that the White Tower can translate Saurian. That is because I'm still not clear on which language that the Sacred Plaques are carved in. It's either they are written in Saurian, or they are written in the script of the Old Ones. Morelian's description of the language matches Boney's description of the Old One scrolls a lot more than 4th Edition's Saurian. Boney has read at least part of the trilogy.
Ulthuan has had four thousand years more years than Laurelorn to find more examples of both Old One and Saurian script. And the White Tower has driven Ulthuan to accumulate all knowledge unto itself for two thousand years. It has a
lot of incentive to translate it.
Why would it? Dwarfs already know that elves are untrustworthy. They know for sure that they taught elves it back in the day, before they proved their perfidiousness. The revelation that it was shared outside the apprenticeship line would shock nobody. And could cause zero wars because Ulthuan retreated from the Old World and still rules the waves.
Tale of Metal possesses infinitely more ability to spark wars than confirming something Runesmiths would already grumble about.