Labour's strategy isnt as looking as far ahead as that, they believe they can force an election in the next few months and will vote against the government regardless.
I've heard people say this, but how? Under the Fixed Term Parliament Act, they'd need two-thirds of the House to agree. Even if ALL the Tory Brexiteers voted it for it AND all the DUP, Iit still wouldn't make two-thirds. And the DUP have said, quiet categorically, they wouldn't do anything that would bring down the government. The mathematics just don't add up.