In what sense? You had fairly heavily pro-austerity Tories joining Labour MPs who had left the party because - for the most part - they felt it was too left-wing. People on the centre-left/left can undoubtedly work with Tory MPs on issues where cooperation is needed and beneficial, but if you're joining an actual political party that's filled with Tories, and which is led by an ex-Tory who agrees with Tory economic policy for the most part, then how can you, in any way, claim to be left-wing or centre-left? It's fairly obvious at that point you're not. Hell, Chuka and co even had the option of defecting to the Lib Dems back in February - they instead opted to join a party filled with Tory MPs, after years of being frustrated at being called Red Tories.
There has always been a further left than the Labour Party. So how can anyone who ever voted Labour claim to be left wing when they didn't join the Socialist Workers Party instead? You could reverse the question and ask why would anyone who believes the mainstream Labour Party is too right wing for them, ever join it in the first place?
We know why though, they couldn't get elected when they stood on their own policies in their own distinct party.
So they move into the mainstream Labour party, change that party and its policies beyond recognition and then claim everyone who won't vote for it is a Tory. That is a poor analysis based on party name recognition not policy or logic.
We are where we are and now and we will test whether politics has changed or not at the next election.The previous evidence and thinking is that extreme left wing policies well beyond the mainstream don't win majorities.
Things have changed and it might be that the old thinking is wrong but if Labour lose the next election it isn't because all the old Labour voters were Tories all along but just didn't know they were, it will be because the Labour Party has become the socialist workers party.