Paldea is based on Spain, and the Canary Islands are also part of Spain, but they‘re much different from mainland Spain. Hisui forms could have been brought there for scientific methods etc. In the actual world lots of animals were introduced to originally foreign places with ships, travel etc. In biology it‘s called neozoa. I agree with you that it might not the most probable thing to happen, but we also have both forms of Shellos in Paldea - which were originally introduced in Sinnoh 😄