You suspect he's too quick to pattern-match every mention from a single example?

Admittedly, this is speculative, but from his tone I did get the impression that he was prone to such matching.

Perhaps this is because I have an unfavorable view of governments in general, but it seems that for an even slightly cynical student of history assassination attempts on rival heads of state by a government should not come as a surprise, especially as monarchies were replaced by democracies.

Actually, a very strong taboo against such assassinations follows from a very cynical theory. Namely, it provides for a convenient Schelling point for national leaders, where they can otherwise escalate war as much as they like without fear for their personal safety. (As long as they don't let themselves get totally conquered, of course.)

But more importantly, who are all these heads of state supposedly assassinated under orders from rival governments prior to 1939? Can you name any attempts of such assassinations in the period of, say, one hundred years preceding 1939? Or even just cases where the culprit is unknown, but a plot directed by a rival government seems plausible?

(The closest example I can think of is the killing of Engelbert Dollfuss that kicked off the coup attempt in 1934 by the Austrian Nazis, who were clearly acting in concert with Berlin. But even that was an all-out coup attempt accompanied by an armed Nazi uprising across the country, so not really an assassination plot, and also symptomatic of the new and unprecedented wave of political gangsterism of which the British government was not a part.)

Given this history (or rather a lack thereof), do you think that it was possible for a non-biased observer in 1939 to view the accusation against British government plot to assassinate the German head of state as unsurprising if true?