The we 'don't know when' line is just one of the standard responses banks social media teams have been given to say.
They've got to know the exact date to get prepared technically to handle all the transactions and sign ups and to have staff trained to help. Same with launch partners you've got to know in advance to train the staff so they are ready for people paying with their phones from day one.
I had a bPay band from Barclays last year and loads of stores hadn't been told about it so you ended up having to explain it was legit..
Also I wouldn't take everything a company says on Twitter to be the whole story. For example I've seen tweets from Tesco saying they don't plan to support Apple Pay:
https://twitter.com/Tesco/status/615889672917659648
But my local Express already supports contectless so will support it (unless they turn it off completely)