This is not as silly an article as it may appear to be in hindsight. When it appeared (in May 1940: https://www.scientificamerican.com/article/dont-worry-it-can...), most scientific authorities, including such non-idiots as Niels Bohr, believed that, while an atomic chain reaction was theoretically possible, the only way an it could be achieved was by bringing together a quantity of nuclear fuel so large as to make the effort impractical.

The first proof that this was not in fact the case came in the Frisch-Peierls memorandum (https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memoran...), which had been written two months before Harrington's article appeared, but which was highly classified and thus out of reach of pop-science journalists. Frisch and Peierls' figures indicated that a critical mass could be achieved with just one kilogram of uranium-235; this turned out to be an underestimate, but not so much as to invalidate their fundamental point, which was that criticality could be achieved with a mass of fuel small enough to be delivered by a 1940s-era airplane.

This came as news to the brightest lights in the world of physics, up to and including Albert Einstein, so it's probably unfair to expect a contemporary writer for Scientific American to be further ahead in the science than they were.