dragons

This is probably a stupid question so sorry i apologize before hand.

It has been said that dragon-fire could melt and consume the Rings of Power, but there is not now any dragon left on earth in which the old fire is hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring

If not even Ancalagon could harm the one ring why has it been said that dragon fire could harm, or am i just stupid and the passage is saying that theres no truth in that since Ancalagon couldnt even harm it and he was lord of the dragons.