Imagine pretending that it's a binary issue. The simple fact of the matter is, that to repair the damage Cameron et al has inflicted on this country, we all need to compromise somewhat. Labour's manifesto clearly states that the party will respect the result of the referendum, and will negotiate a deal that guarantees a level of alignment with Europe on things like workers rights and environmental protection.
What does workers rights actually mean in this context?
As for Labour's 'plan', I'm inclined to think their customs union plan is designed to pretend like they're upholding the referendum result whilst also knowing full well it has no chance of passing, whether because it goes against EU rules as they stand today, or that it would prevent the UK from concluding independent trade deals of its own (which is the main issue with the backstop).

