Aeldari have daemon infested webway here and there, yet none of daemon poor out of those ways into reality, they even collapsed some if push become shove. Meanwhile, the one under Terra require Big E and Astronomican to keep it shut.
What’s so different. To me, this felt like Big E tried to do plumbling himself and have to sit on the broken pipe to prevent it from flooding the house. Better call a real plumber like those Aeldari.
I think the idea is that (before Magnus did a whole lot of nothing wrong) they could have sectioned off parts of the webway and expanded slowly. The Imperium is all about expansion, they always need a new warzone to conquer.
Once they figured out how to make an entrance, they could figure out how to close passages and open new entrances. A safer method of traversing the galaxy.