I mean there are a few minor differences between Trump and Hitler, such as being personally in charge of a huge private army, living in a country that has been a democracy for less than 15 years in which a large portion of citizenry are longing for a return to the glory days of authoritarianism, leading a new and upcoming party instead of joining an old and very established one, competing against a disunited and ineffective political opposition, having a large number of sympathetic officials, politicians and judges who gave him a slap on the wrist prison sentence for personally leading a literal armed insurrection against the country, and publishing a book in prison about how it's his country and race's destiny to conquer the east, extinguish Poland and secure a vast, self sufficient territorial empire from Russia and take care of those troublesome Jews.
The comparisons seem implicitly flippant because Hitler isn't famous for being just some random, crazy, lying politician who wasn't taken seriously but then managed to claw his way into power. When somebody is comparing a politician's career path to Hitler's rise to power, that's got a lot of unspoken implications. Trump's terribleness certainly stands on its own without any need to draw strained comparisons to the Weimar republic in 1933 and Adolf Hitler. America is not in a similar situation, the institutions of America aren't similar, and Hitler has as many differences as he does similarities to Trump.
P.S. The Rise and fall of the Third Reich was published in 1960. Historians had barely had time to breathe by 1945.