I'm not defending SR specifically Paul, or criticising your decision to ask for proof, but merely saying that JC is playing fast and loose with Brexit. In saying that he will renegotiate a better deal than Boris has, he is peeing in the wind, as any better deal is one that the EU will insist is going to keep us more closely tied than the current one and therefore imo we'd be better off staying in, but of course that doesnt deliver on the 2016 referendum. Corbyn isn't going to state that he will vote against his own deal publicly but I've seen several interviews where Labour MP,s have been asked the question and their response has been "we'll have to wait and see what deal he gets" . It's all just a fudge to having to state his real Brexit position during the general election.
I think that the definition of a "better" deal is subjective. What might be "better" for one person could be the very worst outcome for the next person. And in that respect, using the term "better deal" is just spin.
Would JC vote against his deal, and to Remain? I don't think he will. Its been reported numerous times that he has been at odds with many of his shadow cabinet. He's a Leaver. He wouldn't vote for Boris' deal as voting for anything Tory is just something he wouldn't do but I do think he'll vote for his own deal. Maybe he'll hope the Tory Brexiteers will support that deal... who knows.
A fudge, not wanting to clearly state his Brexit position. He's sitting on the fence in the hope Labour will hoover up more of the floating voters.