If Johnson truly believed that the opposition parties were making an “extraordinary political mistake” he would have kept quiet about it and let them make that “mistake”.
This blustering rhetoric is just him trying to goad parties into opening the only escape door that he currently has. If the opposition sit tight and leave Boris to hang until the EU summit on October 17th he will either have to ask for an extension (by law) or resign if he refuses to do so. The conservatives can then elect a decent and truthful party leader, if their blinkered bunch of elderly party members can learn from the horrific voting mistake they made recently.
If BJ has any sense he will sack the very dangerous Dominic Cummings straight away and then apologise to the 21 sacked MPs (who were got rid of at Cummings insistence) and reinstate them. This might go some way to appeasing the huge anger within his ranks at the way a handful of extreme right wingers and one anarchist are tearing the conservative party apart.