Also, I agree with Togo at least in as much as he calls this all "daft". It is daft that a PM with such a significant minority will not be removed by a No Confidence motion and it is daft that he is therefore being kept in power by the Opposition, whose normal objective is the exact opposite.
But there is at least some logic to the Opposition's position, however warped it ends up being. Calling an election right now hands back the advantage to Johnson, in whose gift it still remains to set the date of an election (at least if called under the Fixed Term Parliament Act). He could, and presumably would, set a date that suits him best, which would probably be after October 31st. Moreover, calling an election now automatically dissolves Parliament over the same period that they have just fought so hard to keep it open. It would be self-defeating.
On the other hand, for Johnson to call an election right now shows that he cannot take the negotiations seriously either. Johnson would be too distracted by the election campaign to focus on the negotiations, and, as a further point, it would be certainly impossible to implement the terms of Section 13 of the EU (Withdrawal) Act 2018, which states that the withdrawal agreement can only be ratified with further primary legislation. An election prior to October 31st, called now at least, leaves no time to negotiate a deal, ratify it with Parliament, pass the legislation to approve it, and then pass all other secondary legislation needed to support that implementation.