Think of it in terms of chess moves. Whether Boris intends to negotiate or not is immaterial. The EU has said it will not negotiate. If Boris does not ask for an extension before the 31st of October we are out of the EU. That is already set in law.

To change that law, Parliament has to pass some sort of new legislation. If it looks like that is about to happen Boris can then threaten a General Election. Members may decide to defy the threat and an election is called. October 31st passes and we leave the EU by default, or Parliamentary Members don't create new legislation and again October 31st passes and we leave the EU by default.

There may well be other possible moves like a vote of no confidence, but that would result in a General Election and October 31st would pass with the same result. All Boris has to do is dance between, or fend off, the various options and we will be out. Hooray!