They aren't saying developers are unproductive. They are saying teams of developers are unproductive. In the words of my great grandfather. "one boy is one boy, two boys is a half a boy, and 3 boys is no boy at all"

I don’t know. If someone told me “there is nothing less delicious in this world than your turkey casserole” and you came along and said “they aren’t saying your turkey casserole is undelicious,” I’d be inclined to believe you misheard something.

the difference in productivity between 1 and 2-person teams isn't in 100%, but in maybe 50%. The more people you add, the smaller the productivity gain.

From my own experience, my most successful products have been built by one single full-stack dev. Who did everything

It may be true, but real skill lies in realizing the moment it stops being true which is inevitable with growth and act accordingly.

Team / interpersonal communication overhead, planning overhead, and dealing with less skilled members of the team can easily make 4 engineers roughly as productive as one very skilled engineer, just at 4-5x the cost once you include the increase in product/project management required by multiple people. We tolerate this generally because eg upskilling peers is important to scale a company, but pre-PMF it's a huge advantage to have just the one outstanding engineer.

yes. this is what I meant