But the point is, the "edge" we see here is completely fictional. It's a sharp border in space, marked in purple. Nothing of the sort exists in the real galaxy, so there are no rules for where it might be located.
Still, I agree that "within a sublight journey" is probably way too close...
What I still don't find credible is that Earth would have had FTL technology before Cochrane, or that impulse drive in any of its forms would be capable of exceeding lightspeed. Better to explain the about two TOS ambiguities otherwise: "The old impulse engines" on the Valiant
as a secondary rather than primary drive system for the old tub, "Her power is simple impulse" on the Romulan cloakship as an understandable error of judgement.