So two things here.
First, I mentioned the "fix" in Dungeon of the Mad Mage if you want to create a steel dragon. Basically, you just use the silver dragon as a base, and replace its breath weapon with a line of acid and its cold immunity with acid immunity. And although it doesn't give directions for a song dragon, given that song dragons are often compared to coppers, just use the copper dragon as a base and replace its breath weapon with a cone of lightning and replace the acid immunity with lightning immunity.
Secondly... wait, that means there's already a canonical steel dragon in 5e! So if you don't like the "folklore" line from Fizban's, you can use the fact that a steel dragon appears in Dungeon of the Mad Mage as evidence that that rumor is incorrect.
(I'm assuming that Wyatt or whoever wrote that sidebar was unaware of this steel dragon's existence)