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I suppose you're right about it being the 'best of both worlds' in that there was neither a mass exodus from Labour for being a party of leave, nor was there a mass exodus from Labour for being a party of remain (as arguably there was in 2019 although this was of course only one factor among many in the defeat).
Either way, Brexit was less of a wedge issue in 2017 than it was in 2019. I don't think the idea that 'it was passionate remainers with nowhere else to go' who contributed to Labour gaining seats in 2017 is a particularly coherent one.
It was my recollection at the time though - labour were still benefitting from people wanting "not this" about the Tory plans; being officially pro-leave probably meant that couldn't as easily be used against Lab in leave areas either. So they may have got the best of both worlds (although I recall at the time it being argued as the worst of both too)