I read the parent as saying he is against Brexit (all in on "no Brexit"), presumably for the reasons you articulate.

Correct. More precisely I meant brexit not happening at all. That is article 50 will be rescinded and UK will return to status quo.

All in on "no Brexit", to me, sounds like "going all in" on a round of poker - meaning you're very much for that.

Yes exactly. They want Brexit not to happen at all. Your reply seemed to be talking about "no deal Brexit" which is essentially the opposite scenario.

Yes. He is very much for "no Brexit", ie not doing Brexit. Not to be confused with "no-deal Brexit", which is doing Brexit without a deal.

Yes being very much for ‘no brexit’ means you’re against brexit.