The Labour Manifesto was quite clear in 2017 that they wanted the UK to see through Brexit, including running our own trade policy. They laid out considerable details about the features of a global trade policy they wanted for us, which clearly ruled out staying in the or a customs union. Many pro Brexit voters voted Labour because they stood on a pro Brexit ticket.
Of course an Opposition party can change its mind. This particular change of mind has two big downsides for Labour. one is pro Leave voters who voted Labour will not be happy with this. The second is there is no obvious offer of “a customs union” on the table from the rest of the EU. They have told us we have to leave without cherry picking, so why would this much debated variant be on the table? Both Leave and Remain campaigns, and the UK government., told us before the referendum that leaving the EU meant leaving both the single market and the customs union. I don’t see that anything has changed since then.