I think this is where the defection of yer Bergers and yer Umunnas to the Lib Dems has helped too. Given their stance on Corbyn, I'm fairly sure they would only have countenanced joining the LDs with an explicit assurance that the LDs wouldn't endorse Corbyn either in a Government of National Unity or as any kind of coalition partner.
Of course what this also means is that despite their entire shtick being about stopping Brexit, their anti-Corbyn policy is very likely going to enable Brexit by splitting the remain vote.
And millions wanting to stop Brexit will still vote for them. Thick c*nts.