Labour’s emergence as a Remain party was not a deciding factor in its election night loss. Labour did not only lose votes to pro-Leave Tories, but it also lost votes to the hardline Remain Liberal Democrats and Green Party. If Labour’s decision to go against the 2016 Brexit referendum result and campaign as a Remain party alienated the electorate, the lion’s share of Labour’s lost votes would have gone to the Conservatives or Brexit parties. This was not the case; Labour haemorrhaged votes to all the other parties. 
It is true that Labour lost around 900,000 of its Leave voting supporters. However, it lost 1.1 million its Remain supporters, with many going to the Liberal Democrats or the Greens. 
This exodus of Remain supporters did not have an issue with its Brexit stance but still saw the need to vote with another party in 2019.
Additionally, most polls leading into the 2019 General Election had the majority of the British public wanting to remain as part of the EU. If most of the country wanted to stay in the EU, why would a pro-Remain party take such a drubbing over its stance on Brexit?