They said "can actually deliver", not "could technically possibly deliver".
They could have actually delivered the deal had they voted in favour -- and it would have been better than no-deal -- but many leavers voted against.
Because they can’t agree on D. So they end up with N.
Worse, some of them seem to be deluded into thinking they can get some sort of D+ or D* or D’, even though the EU has made it clear they will not renegotiate.
I believe one MP likened that scenario to the likelihood of Scarlet Johansen riding in on a unicorn.
I don't see the logic here. The deal (let's call it D), is clearly worse than staying in the EU (let's call that option E). From that relation D < E it does not follow that no-deal (N) is better than a deal, i.e. D < N. It may well be that N < D < E. So I don't see how no-deal is the "best the brexiteers can delliver."