'Dúirt sí tada' is another appropriate form of 'she said nothing' and should therefore have been accepted.
There's a difference between "She said nothing" and "she didn't say anything".
Why isn't it "níor"?
Abair is one of six irregular verbs that uses ní rather than níor in the past tense. (The other five are bí, déan, faigh, feic, and téigh.)
can I use labhair instead of abair? Nior labhair si
And does "duirt" not get lenited because it's a DenTal DoTS situation--?
dúirt isn't lenited because it's an irregular verb, and it doesn't follow the same rules that regular verbs follow.
It's not a DeNTal DoTS situation because the letter before dúirt isn't d, n, t, l or s.