I think OP was not asking why we have the 'n' inserted, but why we are using an article at all, instead of 'an artificial intelligence' why not just 'artificial intelligence was used', after all, 'intelligence' is not normally a count noun.

Looking at OP's post history, I doubt they're confused about using 'an' vs 'a'.

In short, I think the use of 'an' here means it's a single specifically trained AI. It would not be wrong to say 'artificial intelligence was used' though, it just doesn't highlight the dedicated nature of the AI used.

Similar difference to 'Someone swapped the faces' and 'An expert swapped the faces'.