Watch the Battle Frontier series and the 8th movie again; TPCI pronounced Bonsly correctly as Bon-sly, it's 4kids who pronounced it wrong as Bon-slee, you got it completely the wrong way round. Heck, Bill Rogers, the TPCI voice actor for Brock, when he was a member of the Serebii forums explained why TPCI's pronunciation of Bonsly was the right one and that it was a "sly bonsai tree"
TPCI knows the correct way to pronounce the English names of Pokémon because they are the American Pokémon company after all, 4kids, meanwhile, has been prone to saying Pokémon's names wrong. E.g; (not English name, but a Japanese name kept in the English version) Raikou, Japan and TPCi say it as "rai-koh" but 4kids would say it as "rai-koo".