I am aware of what the post-briefing says. They do clearly say that the echo is the shield against Tempering; however, the game seems to contradict this statement with the words and implications of other events and conversations in the game.
It's a fact that Thancred is said to be somehow more susceptible to the influences of Primals after his run in with Lahabrea. Yet, Thancred is never stated to be blessed with the Echo in the first place, so how is he somehow more susceptible? It doesn't make sense, but they exclude Thancred from a lot of Primal related content after MSQ because of fears that he could somehow be tempered... as if that couldn't happen to the rest of them just as easily.
Also, Louisoix, as far as I know, is never stated to have the Echo. He seems to get a blessing of light, allowing him to go super saiyan, but as you stated that's not the same thing. Yet, Bahamut does not temper him before he gets a fist full of super saiyan. He only does it after. Also, the company of Hero's are said to have lured Leviathan into a cove in order to defeat him. None of them are said to have the Echo, but none of them were tempered by the lord of the whorl. Further still, Alphinaud and Alisaie both stood against Bahamut with no fear of tempering... so, what gives?
My point is that, despite them being crystal clear about the echo being required for resisting Primal influences, there have been countless people who have come across primals and not been tempered with no proper explanation as to why. Ifrit displayed quite clearly that it is a quick process (it took him, like, a second to brainwash an entire squad of guys), so time restraints or improper preparation is not a good enough reason.
There are only two possible explanations, either 1, SE has seriously overlooked their own lore and failed miserably at implementing it consistently in this regard, or 2, there is more to resisting Primal influence than echo alone. Despite them not saying it directly, it would seem that a committed will to resist is enough. Otherwise, Alphinaud, Alisaie, and the company of heroes would all be mindless slaves to their respective primals.