Ok, so I tested it further and can confirm that harvesting can be done by a fleet (at least a fleet of cargo ships). I apologize for making this post. As I said, I was told that it didn't work by several veterans and I thought I saw a fleet of mine not successfuly harvesting ore, but it was a while ago, so I may have just done the math wrong and they were working properly.

In any event, disregard this thread. Thanks.