Could one use the definite article with 9/11?
But not in your sentence.
The author traces the origins of modern terrorism back to the late 19th century, but he admits that (the) 9/11 was an important event in this long history.
It would not be correct to use the definite article here, because 9/11 is being used as a proper noun. This is shown by your use of event and not date. It seems Tomasc Pludowski is also referring to the event, and so he is also incorrect in his usage of the before 9/11.
You ask other questions:
Will "the 9/11" sound strange to native speakers of English, or will it look like a minor error or just like a variation, an author's choice of article?
In both your sentence and Pludowski's the 9/11 both sounds strange and will be taken as an obvious grammatical error. Whether a native speaker would classify it "minor" or not depends on how you define "minor". It is not one that reputable publishers in the US or the UK would allow to go uncorrected. It sounds glaringly wrong. It is not a variation or stylistic choice.
Could one view the 9/11 as an elliptical variant of "the 9/11 attacks"?
No. Not even when you have referred to the 9/11 attacks previously in the same text. This is because many more things besides "attacks* were part of the event. You could use such phrases as the ones on 9/11 or those of 9/11 to refer to the 9/11 attacks.
For 9/11 (and its variants such as Nine-Eleven and 9-11) as a proper noun or name, see the following:
- American Dialect Society's Word of the Year for 2001 (link)
9-11 Terrorist attacks of Sept. 11, 2001
- Global Language Monitor (link)
The first case is the use of 9/11, itself, as a shorthand for the 2001 terrorist attacks. Using various web metrics, 9/11 outpaces any other name, including the spelled out ‘September 11th” by 7:1 margin.
- MacMillan Dictionary BuzzWord (link)
September 11th 2001, the day when planes flown by terrorists destroyed the World Trade Center in New York and damaged the Pentagon, killing thousands of people
Read the entire piece, as the blog makes it clear that 9/11 is used as a word and not only as a date.
- alpha Dictionary (link)
Meaning: The terrorist attack on the World Trade Center in New York in which 2,993 people from 90 nations perished on September 11, 2001. Terrorists also struck the Pentagon in Washington but failed to reach one other unknown target on the same day.
You could use the definite article with 9/11 in the same situations that you can use it with other proper nouns and names. This is to specify further.
'You had lunch with Beyoncé, you mean the Beyoncé?!'
'I'm confused. Are you sure you're talking about the 9/11 and not any old 9/11, because I remember things differently regarding it.'
We can also use the when talking about different manifestations or experiences of it:
The 9/11 you went through was much more personal than the 9/11 reported on by most newspapers.
9/11, much like Pearl Harbor, is also used as a common noun, and in fact a verb ('I am going to 9/11 your ass all over the place'); see Urban Dictionary for other examples.
The two events are related, of course, as attacks upon US soil that killed in the thousands. A Pearl Harbor is now
a(ny) surprise attack often with devastating effect
See Merriam-Webster online dictionary, where this is the first definition, before that of the place name.
There is another definition for Pearl Harbor that applies equally to 9/11 as a common noun, found in International Dictionary:
A seminal dramatic event that unites a community and arouses it into action against an enemy
This is for Pearl Harbor, which has now been converted to a common noun, that is, one that refers to a class of events. But as per Urban Dictionary and the main stream press, we also use 9/11 in this way, such as
"Such-and-such country has not experienced its own 9/11."
The proper noun 7/7 refers to the terrorist bomb attacks of July 7, 2005 in London.