Yanbu = you're right, they do think they'll lose
Yabu = no, they care what the public think and will try to win the next election
Labour have a very electable leader, in normal times (no brexit/coronavirus/financial crisis) Labour and the Tories almost take turns running the country, if this was normal times it might be Labour's 'turn' anyway. We also have the absolutely shambolic management of the covid crisis, Brexit would have taken away a whole load of our rights and a great many people who voted for it will have died, plus facing a covid recession made worse by 'no deal ' . Even before the Cummings affair, they will have worked this out and known things didn't look good for them in the next election. This frees them up to do whatever they want for the next four years as they don't need to care about what the voters think? This might also help explain the two fingers to the public over the Durham trip.