Your heart is in the right place. Think of it like this, many spiderlings balloon themselves many miles from where they originated, even crossing the ocean, they have been found as high up as 40,000 feet. Even if that was before latrodectus evolved, by the time latros did evolve, the two continents wouldn't have been all too far apart yet for this to have occurred.
I do not think they cross oceans. That’s pretty extreme. Saying that they balloon for miles is all I would believe. Also, the highest point on earth is barely 29,000 Feet. Latrodectus arent Found at 40,000 feet because that place doesn’t exist short of aircraft.