It is not how I would have played the game, but I don't see it was a stupid blunder.   The options before Labour were to do nothing, call for a standard confidence vote, call for a general vote on whether Johnson should stay, or call for a no-confidence vote in Johnson. They opted for the latter. They would not have been granted the general vote on Johnson, because there is no requirement such a motion be granted in any timescale.

My personal preference would have been to call for a standard confidence vote and then at every opportunity during the debate point out that a loss did not mean a general election must be called: since the Conservative party is large enough they could go to the Queen and say Raab or someone had the confidence of the house and so could form a new Conservative government for the interim.   That is, throughout the whole debate keep emphasising all those voting for the confidence motion are really just voting to keep Johnson in position, not the party. There would be no general election.

But that is risky. The whole point of the vote was to get MPs to vote in favour of keeping Johnson in power, and without the explicit mention of him, Labour would not be able to say 'your MP voted to keep Johnson even after he was forced out' on campaign literature. That will be much harder without the reference to Johnson in the motion, but it is still just about possible.