It doesn't matter if western countries mistreat their people sometimes, that doesn't justify mistreating the Chagossians or anyone else for that matter.
Even if you think 200 years isn't a strong claim to a nation, that doesn't change the fact those people lived there and were forceably removed from the islands.
I'm also curious, what is your cut-off point for a "strong" claim to a nation? 250 years? 500 years? 1 AD?