The researchers could not be sure of this result before they did it, which is why I believe there's a reasonable case this should be considered GOF work even if

The researchers could not be sure of this result before they did it, which is why I believe there's a reasonable case this should be considered GOF work even if the lethality of their chimera (in hu mice) was ultimately less than that of original strain. Furthermore, strictly speaking I don't believe we know the actual severity of this in humans. While it's possible to make a fairly confident guess, surprises are still possible.