I don't think there was so much trade between Northern European cities and Arabian ones, in 1350...and yes, some travellers returned with tales about the emptiness of European cities, but Arabian nations don't have enough informations to make that claims.
The TTL's Plague changed the North Europeans-Arabian relationship to an like OTL's China-Europe one during the same time: The Arabians don't know much about European situation. Maybe only Mamluks have that kowledge, as they ordered large-scale explorations in Europe, but Josef didn't express the intentions of giving that knowledge to other nations.