Is he wrong though ? This is what Varadkar has said,

"Meanwhile Leo Varadkar, the Irish Taoiseach, suggested the idea of a no deal Brexit was a fallacy because “a no-deal deal” would have to be negotiated anyway."

"Mr Varadkar cast doubt on whether no deal was a viable concept, telling the Irish parliament: "Nobody knows for sure what would happen in a no-deal scenario where the UK crashed out of the European Union without a deal.
In such a scenario Mr Varadkar said negotiating teams would have to try to agree an arrangement to avoid a hard border.
"Nobody knows how long that no deal would last," he said.
"It's my view that if we did end up in a no-deal situation, we would find ourselves having to negotiate a no-deal deal quite soon thereafter, so it might happen for a few weeks."

So it seems like Varadkar agrees with Davis, if we have to negotiate a no deal arrangement, then it's perfectly feasible that some sort of transition period could be negotiated as well.