It seems clear that a culture doesn't actually have to use, or even have, warp technology. If it's aware
of the existence of same, that appears to be enough.
For example: the Ba'ku in Insurrection
. They obviously had no starships or warp drive in active use, but they knew about those things anyway. And hidden away somewhere they still did have stores of technology (which they used to try and fix Data).
Who knows, the same thing might have applied to the Capellans. They clearly knew enough to receive alien visitors and negotiate with them. So it could be that in some cavern somewhere, they had some kind of technological base...