Just to review and to make a few other things clear:

It IS, in fact, possible to get past the centaurs guarding the Healing Springs. It may take throwing something at them the likes of which they've never seen before, and it may be quite unlikely, but it is possible. Obviously, "giant ikki bird" qualifies.

It is possible once and only once, that is, as they quickly adapt to things like that. It's their job, after all.

And by "only once", that applies right up to when time itself ends. Meaning that absolutely nothing large and flying will ever get by the centaurs again. A dragon the size of Central Spiral itself could try and the centaurs could dispatch it.

Dragons and other naturally large flying creatures have different concepts of life and death and have never had reason to attempt to invade the Springs.

Dwarfish flying contraptions are rather a bit unweildy and fairly easy to knock out of the sky by well-trained, borderline immortal centaurs armed with divine weaponry.