What lore do you ignore?

[Iscah]

I might be wrong or mistaken, but isn't the material used in the Garlean Radios that provides their anti-tempering powers Giantsgall? And isn't that same material used in the process of converting Vrtra's scales into protective talismans?

I would need to comb through the text to figure out if that's actually the case.

Yes, I do remember that being implied. IIRC it's that the ore is from Corvos which is close to (and I think elsewhere stated to be geographically similar to) Thavnair.

So if it is giantsgall or similar, it's possible that the radios have a small amount of power to protect from a single burst of tempering, but it's the dragon scales that give it lasting energy to protect you in a danger zone, or something like that.