Well. If you want to be pedantic... the root of the bson document is.. a document (object), not an array.
So strictly technically this should be an object, not array.

But yes, working with stdclass is just unnatural in PHP, so we want arrays instead.
Supporting both array and stclass in zpp is a pain in 5.2. Only accepting array makes your life easier - and makes more sense to a normal user :)