a or an

Yes, it should be "an." The use of the indefinite article is always determined by the sound of the word which it precedes, no matter what part of speech that word is. So, one might say: "It was a day to remember, it was so awful."--but: "It was an awful, an unforgettable day." In both of those cases, "an" immediately precedes an adjective, but adjectives which begin with a vowel sound.

This is exactly the case in your quote. The acronym is used as a adjective, and begins with a vowel sound, so "an" is required.

https://en.oxforddictionaries.com/usage/a-historic-event-or-an-historic-event

People often believe that they should use the indefinite article an in front of words like historic, horrific, or hotel. Are they right or wrong? Should you say ‘an historic event’ or ‘a historic event’?

An is the form of the indefinite article that is used before a spoken vowel sound: it doesn’t matter how the written word in question is actually spelled. So, we say ‘an honour’, ‘an hour’, or ‘an heir’, for example, because the initial letter ‘h’ in all three words is not actually pronounced. By contrast we say ‘a hair’ or ‘a horse’ because, in these cases, the ‘h’ is pronounced.

Let’s go back to those three words that tend to cause problems: historic, horrific, and hotel. If hotel was pronounced without its initial letter ‘h’ (i.e. as if it were spelled ‘otel’), then it would be correct to use an in front of it. The same is true of historic and horrific. If horrific was pronounced ‘orrific’ and historic was pronounced ‘istoric’ then it would be appropriate to refer to ‘an istoric occasion’ or ‘an orrific accident’. In the 18th and 19th centuries, people often did pronounce these words in this way.

Today, though, these three words are generally pronounced with a spoken ‘h’ at the beginning and so it’s now more logical to refer to ‘a hotel’, ‘a historic event’, or ‘a horrific accident’.

The letter "h" is not pronounced with a "spoken h."

I wonder if there's some flexibility with regard to spoken vs written language. I know that when I'm reading or writing I'll use "it was truly a historical event". But if I'm speaking I 'm apt to unconsciously drop the "h" and automatically say "it was truly an -istorical event".