At the time of the adventure, neither side has a clue, so there is no point in cooperating. Thus, Tebok's "Only if it's convenient and appropriate at the time": the time is not now.
How does that make any sense? Why wouldn't NOW be the better time? The two factions are dealing with a threat unknown to them that could still be around and maybe heading towards it's next target, and the Romulans decide to shrug it off just so they can show the Federation who's boss? And if the time isn't appropriate, why not just tell Picard that? What was there to gain by agreeing to cooperate, decide not to cooperate, declare "We... are back" and than leave?
For being the introduction to the Romulans, having them do nothing except waste everyone's time is not the most intimidating move to make.