There is no deliberate misunderstanding but the figures are available by constituency.
I think it therefore fair to ask how you think a Party that campaigned for and had a manifesto of Leave (or at the minimum, vowed to respect the referendum result) and which has now transformed into a Remain party will fare in a General Election, given that 90+ of the constituencies it represents voted leave - many by a large majority.
Do you not think this could cost Labour dearly in a General Election?
There is a deliberate misunderstanding, and that misunderstanding is dividing the country into Leave vs Remain camps without taking any account of anything else.
There are areas - the majority of those 90 seats - that voted Leave largely because they've been devastated economically for the past fifty years. If Labour were still a party of pro-EU, being "pro business" (ie: pro agency work, pro outsourcing etc), pro-"fairness" and basically giving not two figs about their core vote then yes, they'd probably see their vote share go down (as they did in 2001, 2005, 2010 and 2015) and they'd probably lose some of those seats.
The problem Boris has is that Labour are not that party any more, visibly.