I think people are overplaying the significance of this Brexit party move.... at this stage at least.
Firstly - the chances of any BXP MP’s have pretty much evaporated. So this couldn’t help Johnson later on form a potential government.
Secondly - A Tory majority assumes that they hold onto all their existing seats and take them off Labour. However moderate Tory voters have now seen their party align (and in their eyes become) a Hard Brexit party. This will make them vulnerable to Lib Dems in those marginal seats, and to the SNP in Scotland. Some Lab/Con marginals that are not in heavy leave areas, areas of high cultural diversity could also be at risk (Northampton North is an example here)
Thirdly - Although it would appear that it could benefit the Tories in current Lab seats (but with big Leave population) will traditional leave voters go to the Brexit party knowing it will deliver another 5 years of Tories and the misery that goes with it? Some will of course - but maybe not as many as people think.
Labour, LD and others need to respond. They should start with simple seats to stand down in to build a common approach and good will. For example - all should stand down in Brighton Pavillion for Lucas, Lab stand down in Richmond for the LD, LD stand down for Lab in Canterbury. Show voters that want the Tories out that you are serious. And then from there it can be built on.