This is an interesting case, making chimeras can always be thought of as "gain-of-function" research because you really only know if you succeeded after you've created and tested the new organism, and there might be failures along the way if indeed creating a more lethal organism is your goal.

In this case there is some handwaving about 80% mortality vs 100% mortality in a mouse host about it technically not being more lethal than the original. But what if the testing revealed 100% mortality plus some other metric of increased transmissibility or something similar after the fact. It would've been unknown to the researchers at the time of synthesis and fit every definition of "gain-of-function" research.

I think in light of this inability to know apriori if a virus is going to be more/less lethal we're splitting hairs when we say this is not technically "gain-of-function" research.