Tabling a vote of no confidence when you know that you do not have the numbers to carry the motion (because the DUP are not on your side yet, and because no Tory MPs will vote with you), will inevitably strengthen the government and will unite the Tory Party.
Tabling a vote of no confidence like they have done is meant to try and bounce Labour into backing a second referendum now, too early, and is meant to make Labour drop in the polls. Both of those will also strengthen the government.