Clearly, the line meant the two governments had arrived at a point of cooperation.
Cooperation is the explanation that makes most sense. I've seen the spy suggestion, but that has a flaw. The Klingons know everything there is to know about their ships. To steal the plans of an enemy in order to build duplicates of the ship for your own navy is not tactically sound. Since the enemy, the Klingons, know and understand the tech and design, they can easily exploit any weakness, intimately know how each and every system operates, and would even now the deck layout by heart. This makes combat, sabotage, and boarding actions so much easier.
An alliance of some kind is the most logical answer. Either the Romulans bought or traded for the ships outright or for the plans and construction specs.