If Labour had won every single Tory Remain seat (71 I think?), they would still not have won. Johnson simply had more opportunities to overturn Labour in 'Brexit marginals' than vice versa, even if Labour had come out strongly as the Remain Party. Even when probably (are there any stats?) division of votes between Leave voters and Remain voters must be around 50-50 still.
Conversely if they'd come out as pro-Brexit, there's a lot of seats they might've lost.
Good analysis here: :
Labour seats they would have lost if they'd backed Brexit this time: Putney, Canterbury, Portsmouth S, Bristol NW, Weaver Vale, Battersea, Enfield Southgate, Bermondsey, Leeds NW, Cardiff N, Bedford, Coventry S, Warwick, Plymouth Sutton, Sheffield Hallam, Newport W, Reading E....Gower, Wirral W (20 seats)
Potentially: Lancaster, City of Durham, Croydon C, Chester (5)
And the Scottish seats they would have lost (and did lose): Coatbridge, East Lothian, Glasgow NE, Kirkcaldy, Midlothian, Rutherglen (6)...Plus the England/Wales marginals they'd have still lost: Kensington, Colne Valley, Crewe, Derby N, High Peak, Ipswich, Keighley, Lincoln, Peterborough, Stockton S, Stroud, Vale of Clwyd, Warrington S, Dudley N, Newcastle under Lyme, Barrow, Ashfield, Bishop Auckland...Bassetlaw, Grimsby, Stoke N, Penistone, Wakefield, Vale of Glamorgan, Wolverhampton SW, Dewsbury (26)
So that's 52-57 seats Labour would have lost (or not won in Putney's case) with a pro-Brexit policy and Corbyn at the helm - and that's assuming a limited c3% Con>Lab swing.
A bit damned if you do, damned if you don't maybe? And further taking up Remain/Leave as simple positions for electoral gain is the wrong way to thing about it. Do you genuinely believe Brexit will be good or bad for the country? Solve that first, achieve consensus in the party and build policy and campaigning on that basis. Easier said than done but I don't get the sense it was really attempted.