That tells us nothing useful. It’s not as though Leave won by a landslide and for all we know the non-Labour voters that they need to win over in those constituencies (e.g. young people) could be most tempted to jump ship by a Labour party that is pro-EU.
There will be a lot of data behind their approach. I tried explaining this possibility to Oscie. It’s only my assumption of course but I think that if you had a Venn Diagram of voters, the number of Leave Labour voters who would stay at home or switch to vote UKIP/Tory if Labour tried to run on a pro-Remain/2nd Referendum platform far outweighs the number of Remainer-Tories who would actually vote for Corbyn.