His Gene activation is something borne out of association with Germa, which has got nothing to do with the “undying power of love” virtually it’s the opposite . Sanji is attributing his toughness to the “power of love”
It can only be a power emanating from some other source, that’s virtually what it comes down to
I’ve been speedreading this entire arc so don’t remember exactly whether or not the Séraphins have a consciousness so as to understand concepts like “gene manipulation” or CoC, but the S-shark sure looks like he hasn’t seen this shit before