Maybe I'm missing something but I don't get the argument that somehow this de-duplication was bad because "there proved later to be tons of special cases for each shape" etc.

That's why object hierarchies work, right? You have a Shape object with all the basics, then the exceptions that occur in e.g. Oval are isolated the Oval sub-class. Makes way more sense than having Oval just repeat all the boilerplate that's already in all the other shapes.

Deduplication matters a lot. Especially for productivity. (For example, if you repeated the same logic in 10 places, then made a patch and forgot to update 1 of the 10, it would be fine if you have a test suite that catches the error, but it sure wasted a lot of your time!).

The stuff about team work etc. sure, but that's completely orthogonal.