That's not an unfair description, but I don't really understand how we can describe it as "tribalism". What you're describing seems to be, rather, the assumption on that part of the leadership that the behaviour of voters would be far more tribal than it actually was. I also don't think that the dynamics that drove voters from Labour in Scotland would push them towards the Brexit Party. The archetypal Labour to SNP migrant is under 50, completed further or higher education, and living in or around a city. The archetypal Labour to UKIP/Brexit voter is over 50, lacking in formal education, and isolated from urban centres. They represent substantially different constituencies; the only thing they have in common is the vague description as "disenchanted".