It's not just about Brexit though. I don't trust them not to back the Tories on policy in general, or at least not to hamper Labour's economic agenda.
In other times I'd agree, but the Tory party is a very different thing from what it was even nine years ago, when Clegg made his devil's bargain with Cameron.
Really what we're seeing is the effects of party politics dominated by two main parties - one massively right-wing and unambiguously pro-Brexit, the other progressive but fractured between its pro-Brexit leadership and anti-Brexit majority - with possible kingmaker roles to be played by a party that's more or less socially liberal and strongly anti-Brexit but economically right-wing, and another party that's like the Tories on crack.
The Tories have virtually nothing to lose by getting more and more right wing, as that will only help stave off the threat from Farage, whereas Labour sound a lot like they're saying "Who else are you going to vote for, chump?" to progressive voters who don't want any kind of Brexit.