I’m not completely certain I agree with this statement. I’ll tell you why.
First let me state clearly, I am not a fan of Corbyn, nor was I during his leadership. I consider him an inept leader who cost the left the chance of governing with his inability to compromise and deliver effective, election winning communication.
Nonetheless, his performance in 2017 was spectacular. For the purpose of simple analysis, I will use Blair’s election in 97 as a “start point” because it corresponds to possibly the best high water mark for Labour in living memory. With that in mind, we can say that no Labour leader since Blair in 97 accumulated more votes than Corbyn did in 2017, and even in percentage terms the share of the vote was Labours best since 2001. Corbyns 2019 performance, considered disastrous by most, still allowed Labour to register more votes than at every election since 01.
The point? There are plenty of Brits who are not conservative. But, equally, plenty who are. However this doesn’t mean the left can’t win elections. I think the trick is not to motivate those who are conservative to vote… which, in my opinion, Corbyn was very bad at doing.