I was somewhat skeptical of this claim but haven't yet investigated it.
Your approach of showing that "kerning" (or just letter crowding )seems to appear in other known valid documents of the era is one way to proceed.
Another way is to characterize fully the relative positions of all the typed letters on the document. Although the character pair "ap" appears to be kerning, I think I noticed that the lower case "a" seemed to strike closer to the letter following it than the letter preceding it. Perhaps the "p" tends to strike slightly to the left of where intended. The bars on which the letters of a manual typewriter were located were notorious for getting jammed and becoming slightly bent.
An analysis of each letter's apparent spacing relative to each of the other letters might permit one to correct for that letter striking consistently to the left or the right. Once such a correction is made, then one might reasonably assess whether kerning is taking place.
That's a very good theory, and a very good thing to look at.