To be fair, the echo preventing Tempering has been a pretty explicit fact throughout the game. It was even directly referenced with Krile, who they agreed should be the one to investigate the Warring Triad since she was immune to Tempering due to the Echo. Similarly, one of the plot twists regarding Shiva was that Ysale had the echo. While the characters previously though that she might have summoned Shiva and been Tempered, her revealing that she had the echo (the 'hear, think, feel') meant that she was controlling Shiva and not the other way around. Similarly, all of the 1.0 player characters were part of the Path of the 12, and thus assumed to have the Echo. This is why they're the ones to fight Primals most effectively, because they're completely immune. One of the most interesting ideas regarding the Echo is one put forth by Urianger in Stormblood, who said that the Echo might be some sort of hypersensitivity to Aether. If that were the case, it makes sense as an ability that can be imparted by various means, whether through the actions of gods such as and Hydaelyn and Zodiark, or through genetic modification like the Resonant. It would make sense that the gods would be better at bestowing such abilities though, hence Fordola's vulnerability to Aether flashbangs and the other negative physical effects (like death) of the Resonant process.

Either way, seeing Arvenvald in the main story cutscenes is both promising and fun!