"I am not able to say what they ate" should be allowed, non e' vero?
Yes, it should be.
"I am not able to say what they ate" = Non sono in grado di dire cosa hanno mangiato ( i don't know what they ate) . Non posso dire cos'hanno mangiato = i know it but I can't tell you
Now I really want to know what they ate.
Though not literal, why not "I cannot say what they had to eat?" - Ok - I know this is the infinitive lesson and all and mangiare wasn't used, but still.. seems like an equivalent translation, no?
Great, because that's what I said!
"I'm not able to say" & "I cannot say" mean the SAME THING!