Labour voters are something like 65% remain voters.
70% of constituencies with Labour MPs voted Leave.
Yes. But those constituencies also have other voters in them. Hence a minority of Labour voters (35%) added to the smaller number of Tories and others can beat the rest of the Labour vote. Plus the 10% ish of voters that came out for the referendum that didn't normally vote.
Also remember that Europe isn't everything. About 10% of the voters the Lib Dems lost between 2010 and 2015 went to UKIP! So they were previously voting for the most pro-EU party out there. Presumably because they were "anti-politics" voters, surprised when the Lib Dems went into actual government.
One of Corbyn's problems is that the Labour working class voter might see him as a tad unpatriotic. And maybe switch. After all Thatcher did pretty well with working class voters, it's how she got such big majorities.
In 2017 a lot of voters didn't like either party - but voted tactically anyway. Can Labour get back to 40% again this election? I doubt it.