...the IAM has engaged with the NMB but seems to be asking for more negotiation dates. When an impasse is asked for, it means that future negotiations are futile. So seems to be a contradiction,
Literally I think you're right. Reality, however, is something that I believe different. While we may be at an impasse, possibly literally, figuratively, and politically, that doesn't mean that attempts should cease. Were that the case there would be no success once an impasse was declared. Without eliminating the possibility of job actions and/or strikes, it's still possible to be at an impasses and continue to try and resolve differences. Realistically using the term 'futile' in regards to negotiations is a poor choice of words. A more accurate way of defining the situation would be to say that 'at this time no headway is being made.' It's borderline semantics but they do imply different things.