But the militia did have warpships in its possession: in the very same episode, DS9 was assaulted by two or three ships of the same design that had allowed Tahna Los to evade Cardassian pursuit in "Past Prologue", and would be seen in interstellar flight again in e.g. "Rules of Engagement".
It is of course possible that the warp drives of the assault ships, as well as those of the winged interceptors, were out of order at the time. Similarly, the group calling themselves the Bajora had in "Ensign Ro" operated a ship incapable of warp, even though the same design was seen to be interstellar-capable in many other episodes, including ones where Bajorans operated it ("A Man Alone" for starters).
I'd think that it would be a priority for the militia, sooner rather than later, to get the warp drives of all these ships working - if not for interstellar missions, then at least for quicker insystem interception. But it's possible that many of the interceptors still lacked working warp drives at the time of "Shadows and Symbols", as there was no talk of direct military aid to Bajor, and no sign of the planet suddenly getting richer or anything. And even a few ships with hobbled warp drives would justify Kira's exacerbated designation of the formation as an "impulse fleet".
Timo Saloniemi