Treat unit tests as a form of documentation in an executable format. It is invaluable in long running or larger projects.

(As opposed to automated functional tests or pure regression tests.)

Also that evidence was comparing test driven against test later, not against no test. That project tested was small enough and short enough to be ran by a single dev.

That is about TDD and not unit testing in general. And there sample population was graduates and undergraduates.

I'd say 90% of developers, which includes a lot of good ones too, never learned to write good unit tests. Poorly written unit tests are worse for productivity and no better for correctness than no unit tests.