I think that the rudeness in the specific example of the OP is impossible to gauge without actually being there.
But, I do think that the idea that by virtue of being somewhere first in a store, you are somehow in control of that bit of the store until such time as you choose to move on is wrong. I mean, if it's access to a salesperson or a counter, then you are, but if it's just an aisle and shelves, then you have no more right to it than any other customer.
That said, if someone is clearly about to select an item and then move, it would be rude to expect them to allow you to do so first, but not if they are browsing. Of course you should always say "excuse me" and wait for them to allow you access to the merchandise, but the first customer would have no justification for saying "No, we're still looking", so that "excuse me" is just politeness, not a request for permission.
It's impossible to say who was really in the wrong in the OP, because we don't know whether the second customer was being surly or oblivious, and we don't know how long the OP and her husband were browsing for in that bit of the store.