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Votes are private so no one can ever really know. It's all guesswork.
Found some polling which suggests about 14% of Con voters planned to switch to Lab and Con voters were about 12% less likely to be certain to vote compared to Labour. There might be other polls. https://www.survation.com/labour-set-to-win-in-wakefield-by-election/
He's probably wrong that no voters switched and it was just Tories not turning out. Would need to see more convincing polling. Either way if more than half your voters don't turn out I'm not sure you can just win them back overnight
Isn't he assuming that the entirety of the drop in turnout was ex Tory voters?
It could just as well have been that all the 2019 Tory voters turned out to vent and other voters tended to stay at home.
Or more likely, broadly similar proportions of each party's 2019 voters didn't turnout.