Re: DROP TABLE can be issued by schema owner as well as table owner

It is not documented clearly.  Any information not made clear is an opportunity for an error which leads to a vulnerability.

It is not a vulnerability in postgresql itself.  It is a vulnerability in an ill-designed system, which can come about due to misinformation / lack of clarity.

Putting your first sentence ("For a specific object, any superuser, the database owner, the schema owner, and the object owner could drop the object.") in the documentation would remove the opportunity for error.