People are probably just baited by Shanks' tattoo, which might just be a normal one and no abyss mark. He lost his arm to a local sea king in east blue, who more than certainly does not have Gaban's level of Haki. The scar he received from Blackbeard indicates it too, even though you could assume BB had this type of Haki even back then when they were young, which I doubt (though the east blue sea king is the best reference point anyways).
Unless Shanks can actively restrict himself from using the regeneration and intentionally sacrificed his arm the moment he saw Luffy is the one. Though if removing the arm physically resulted in him getting rid of the magic under the hood, he might as well had his mark removed via burning his skin.
I think people shouldn't jump to conclusions yet.