Re: Forget the rational arguments, it's election time!
I don't think that's true. In this case, Boris Johnson is not quite the loony right-winger that he's always been portrayed as. He's socially on the Cameroon wing of the party, and I think economically too - but he's always been considered more on the right because of the Europe issue. So obviously he's promising more spending, because he's planning so spend more. Which was clear from what he said when he first became PM. Whether they really think they can sustain that if the economy goes South is another matter - but then they also think that they want to avoid the economy going South by spending more.
As for Corbyn, he's a Bennite. He was on Tony Benn's deputy leadership campaign in 1981 (or was it 80?). He went into the last election on Miliband's manifesto, maybe because there wasn't time, or maybe so as not to frighten the horses.
But in this election the left have taken almost complete control of the Labour Party structure. I'm pretty certain that this election is going to be about what he believes. He's been a Brexiteer since the 1970s, and so he may be lying about that to stop the entire set of MPs from defecting - but it's not like any of these other views should be a surprise. His main advisors used to be members of the Communist Party! In the 1980s when there was no excuse for it - and all the horrors of communism were known.