I don't think the same argument applies here. The Druchii are famous for sailing around the world, not as much as the Norscans, but still known for it. They aren't a problem for the Empire because Marienburg took the coastline with it in the divorce. But they are for the rest of the Old World. Greenskins aren't really famous for ship quality. So the harm greenskins in Naggaroth will inflict on the Old World is much less than the Druchii can. The Skaven Underempire is renowned for its global reach and how utterly incapable it is of getting along. So dealing a blow to a portion in Naggaroth cannot be relied upon to be more helpful for the Old World than letting the Druchii bleed more against them.
But at the same time, Ulthuan has Waaagh and Peace. It isn't going to leave that knowledge locked up in the White Tower and forget about it. Any of its mages that fight greenskins or expect to will be interested in reading it. So to will many Princes. A Nagarythian recognized that Waaagh and Peace was important enough to attend personally. In the nine years since the lecture it is extremely likely that a Druchii spy has come across it and sent it to their handlers. If it hasn't happened yet, it probably will happen in the next few decades. But the Druchii are very fracticious. It's likely that only a few factions have it, if any do. So there is potential of getting a lot of magical lore out them for something they probably will be getting anyways. I'm sure the Druchii are also aware of that to a degree but it's still an option to keep in mind.
Edit: Come to think of it, I'm not sure if Mathilde would know the degree to which Ulthuan is infiltrated by Naggaroth.
