The point is that threatening to trigger an economic downturn that, at the very least, pulls the UK down as well isn't a threat worth making, and that's what almost everyone accepts No Deal would end up meaning. There's more than just the economic effects to consider, as well.
Sure, you could have tried that, and see how it works, but since it amounts to, from the EU's perspective at least, threatening to shoot ourselves if we don't get our way, I think it justified to dismiss it as a reasonable posture that would have strengthened our hand.