I don't think you're entirely correct. Labour promised to hold a second referendum after making a deal, where rejecting the brexit altogether would be an option. The Shadow foreign secretary even promised to campaign against the very deal she'll herself make, and 100 labour MPs pledged to reject referendum result outright, not even through a 2nd referendum.

Given how much support brexit had in traditionally labour seats in the north, I think, with the benefit of hindsight, labour could have avoided that embarrassing election result if they firmly committed to a soft brexit.