I thought sephardim scattered into a lot of places, including elsewhere in Europe (even places like Amsterdam) and North Africa. Not to mention conversos that stayed put.
I say "even" because the stereotype is Southern Europe, Turkey, or North Africa. The further north you go in Europe, the more Jewish and Ashkenazi identities get merged and blurred in people's minds. So the history of sephardim in Netherlands goes against the stereotypes.
I'd find it hard to believe all the cities in Spain decided to expel the Jewish people all on the same day. The inquisition began decades earlier so there probably were indications something bad was going to happen
They're probably referring to the Alhambra Decree [0], which did expel all Jews by a set date. That date was the end of July, and Columbus sailed on 3 August, so it's very close but not quite the same date.
