This is the first mention of the word "suimiuil" in a multiple choice question. what is he difference between this word and "speisiuil"?
Nothing. They're synonyms.
Go raibh maith agat!
Often the 'sé' is left out in speech.
Too much SH :)
Why is suimiúil not accepted?
my understanding was always that it should be nil sin speisuil, what is being said above seems more to be like 'it isnt that interesting'
Your understanding is wrong. Both níl sin spéisiúil and Níl sé sin spéisiúil are used, but the níl sé sin construction is more common.
"it isn't that interesting" - níl sé chomh spéisiúil sin