Similarly, Jouret in "BoBW" was claimed to be one of the UFP's outermost colonies, yet a mere week's flight away from Earth at most. Quite possibly Jouret in turn was butted up against Romulan space, defining another nearby frontier.Cardassia and Bajor both seem to be relatively close to Earth--about a week away at maximum warp perhaps--but they could still be considered on the Federation's frontier if that particular border butted up against Cardassian space.
One wonders if this was "common knowledge", indicating the Bajorans had had an interstellar presence for a long time, or a tidbit known only to those of the archaeological persuasion...Picard mentioned reading about ancient Bajorans when he was much, much younger.
Apparently, if we take Picard literally Bajor had a city culture half a million years before Earth had one. But Bajorans are also extremely conservative and seem to avoid expansion and exploration - even after those half a million years of "culture", there are still corners on their world they haven't bothered to explore, mountains they haven't surveyed. Perhaps Bajor never ventured to space nor made a name before Cardassia came and picked the ripe fruit? That way, there would have been relatively little interstellar outrage, because interstellar empires are probably annexing "primitive" cultures left and right all the time. The UFP supposedly does that, too; at least they tried to woo many apparent non-starflight cultures in TOS and TAS to some sort of an alliance.
For all we know, Bajor was well within the Cardassian sphere (or non-sphere) of influence back in the days of ENT already. It just wasn't conquered yet, merely englobed. We don't know how long Cardassia has been starfaring, but half a thousand years prior to Earth's entry to the warp club wouldn't sound out of place. After all, we already have to accept that Klingons and Vulcans preceded us to the stars by at least that much. And we further have to accept they didn't conquer or otherwise annex the primitive Earth at that time yet.
Timo Saloniemi