For one thing, nowhere in the episode is it stated that they are pre-warp. All we know is that they resemble mid-20th century Earth in some ways (but we see technologies more advanced than that in use in the episode itself), and that they warranted a Federation visit sixty years earlier already (which either means visiting planets from the time of the Lumiere brothers, Daimler and Edison is perfectly fine, or establishes that this world does not develop at exactly the same rate as Earth).
For another, nowhere in Star Trek is it stated that possession of warp drive would have anything to do with whether to make contact with a planet or not. That's just an urban myth, no doubt also believed by many Trek writers but thankfully never turned into actual onscreen dialogue.
Clearly, the folks on Angel One are familiar with the existence of the interstellar community and its alien cultures from the very start, that is, before Picard and friends arrive. There'd be no point not to contact them, then.
Timo Saloniemi