As long as you have not committed to exclusivity with either guy, I see no problem with your dating guy B.
You do not want to lie because that can come back to bite you, but you do not have to tell the whole truth either.
I don’t think there is a right or wrong as we are all different.
So, went on the date...had a great time.
Glad you had fun and are planning to take things slowly.
And yes, that's why it's important to date more than one guy at first so you're not so invested in any one guy.
Still kinda sucks though that Guy A didn't follow through with his intent to call and set anything up.