Presumably the explosion ejects enough material to bring the remnant below the limit. Or, possibly, rotation allows the remnant to not immediately collapse (the Chandrasekar limit is for a nonrotating body, although I understand the increase for a rapidly rotating star is only a few percent.)
It seems from the wiki that there is no actual collision. This seems to be something between a nova and a supernova.
https://en.m.wikipedia.org/wiki/Type_Iax_supernova