Given a choice, do you think Hitler would have taken to a book or off the cuff remarks and Twitter?
Also, how many terrorists are there which the US could assaninate? Multiply that with 10 to accomodate with a family and you may up with a number not far off from the 10000 Hitler proposed.
Hitler and the NSDAP was way more organized than Trump. The party had a well defined manifesto and their ideological bent was pretty clear. Hitler attempted to gain power through a coup before trying the legit route.
Certainly there is definitely some routes of comparison between the rise of fascism in 1920s/1930s Europe and the right-wing-populist Trump phenomenon. However, in my mind the past politician that best approximates Donald Trump is Silvio Berlusconi. ("You never know", of course, but at this time I think some of the right wing political groups in Europe are better targets for 1920s-30s European fascism comparisons.)
Also, he hasn't written them in a book, it's an off the cuff threat, which makes it less serious, in my opinion.