From what I've seen in game, I've gathered that there are three ways an individual can become tempered:
- The Primal can do it to you personally. We witness Ifrit doing this to a crowd of people just before we fight him.
- You can take part in summoning a Primal. Frixio's dialogue indicates that is why the Touched Sylphs became Touched.
- Beast tribes can capture you and convert you even without the Primal being summoned. Numerous Beast Tribe daily quests deal with rescuing captured individuals before they can be Tempered - and one in particular involving the Sahagin, you are sent to rescue people knowing SOME but not ALL of them have been Drowned and will need to be put down. Since this quest can be performed BEFORE you fight Leviathan in the Main Scenario (canonically, it has been many years since the last summoning), it's clear that the actual presence of the Primal is not required. (Unless they use a handful of crystals to summon an Egi or something.)
I'd never heard of Fern mentioning that the Touched Sylphs asked Ramuh to Temper them. I can't imagine why they'd do so, but even if that is the case it does not necessarily change any of the above.