A smallpox epidemic swept the St. Lawrence and Ohio river areas all the way to Ft. Duquesne in 1755-56. This shows that the siege of Ft. Pitt is not the origin of smallpox in the area.
Again, it doesn't need to be the source of an epidemic to count as germ warfare, but needless to say we're off-course here. The original reason we were talking about smallpox was because Christians played a role in its introduction, but it's true that the epidemic would have happened intentionally or not.
What I would say is like, if 99% of the current population of the USA was wiped out by a pandemic, and then Muslim settlers started coming over and conquering our former land, would you honestly not characterize that as an 'Islamic Conquest'?