As you point out, there's a huge difference between receiving 1,000 rads of whole-body exposure and 300,000 rads of point exposure. That beam was tightly focused. It utterly destroyed everything in its path and didn't touch anything else. (Well, actually, there's always a small amount of other activity inside the tube, but it would not have been significant here.)
Indeed, much of the radiation dose didn't have any effect whatsoever, because the first few milliseconds of the dose had already completely destroyed the tissue. The worst effect would have come from his head moving as he collapse, spreading the dose over a 2-dimensional surface in his skull.
As it happens, physicists in the early days of particle accelerators, when the energy levels were much lower, would align the beam by sticking their heads into the beam until their retinas lit up from the radiation. The first physicist would hold himself steady while the second physicist marked the location. That's why a great many nuclear physicists in the 1950s had thick glasses: the lenses of their eyes took the greatest damage.