What is "as American as April in Arizona" meant for?

I was reading an article that was telling a story about Russian writer Vladimir Nabokov:

Paris Review asked the Russian writer Vladimir Nabokov, "Do you consider yourself an American?" He said, "Yes, I do. I am as American as April in Arizona."

What does he really mean by "as American as April in Arizona"?

Solution 1:

There has been much written by literary critics on this phrase.

Julian Connelly speculates

With this affirmation [...] declared his firm allegiance to the country to which he had emigrated in May 1940 and displayed a salient feature of his verbal art: his creative approach to language and his fondness for rejuvenating dead clichés (“American as apple pie”). 

An article at Review32 gives a hint of an answer ...

[...] The phrase yields, like an aphorism, to the superfluous pleasure of assonance, but it is not superficial. April was the month of Nabokov’s birth, and Arizona one of the sites of his entomological discoveries. America was part of his personal mythology. In the afterword to Lolita, he cites the town of Portal, Arizona, as one of his summer ‘headquarters,’ a recurrent stop along his yearly butterfly-hunting expeditions. It was there that, after outdoor activities, ‘in the evenings or on cloudy days,’ the writing of his best novel in English so far would be ‘energetically resumed.’

Perhaps there is a very gentle touch of sarcasm in the simile since how American can a month be, and clearly Nabokov was a complicated multi-national person so perhaps thought the question slightly silly. This last comment is just a personal opinion.

Other quotes by Nabokov show his admiration for America so the phrase is I think warm hearted and not bitter.

[...] ‘I feel intellectually at home in America.’ And again: ‘I am an American, I feel American’. And again: ‘America is the only country where I feel mentally and emotionally at home.’ -  literaryreview.co.uk