Startups are still far more than just their teams. This is different - the author seems to be arguing for something broader as if they create a shared contract technical organization underneath each of the 3+ ventures that may go in wildly different directions.

A few problems come immediately to mind - people and personality conflicts. Can you imagine 3 CEO typess competing for resources/timing on various projects? It might sound ideally initially but conflicts in time and resources are bound to happen.

The second is trying to convince each startup founder that their idea is equivalent to the other two and the timing/stage of development is the same. I doubt it's an impossible idea, but certainly not an easy one to execute. A lot of (contractual) planning for "worst case scenarios" would be especially important here.