It doesn't need to be absolute, just good enough.
It's the most plausible explanation.
Why can't they smell it? Maybe because of the thing that he has on him so as to not smell human
Occam's razor. It's simple, it's obvious.
Him wear thing make no smell.
So he no smell.
Without it, he smells delicious and causes beasts to wig out (date with Ameri). It was too weak at first and he smelled kinda tasty ("Aberhauge...tzurezaza").
Sure, I'll rephrase: the number of prerequisite assumptions in both cases is the same; the perfume is good enough to mask fresh blood vs. it isn't.
Occam's razor is not a symmetry breaker here. You are applying it incorrectly.
To be crystal clear, your explanation may very well be correct, so you don't need to sell it to me. I'm pointing out that there are other defensible views that account for the perfume given what we've actually been told about it. You might be right, but you also might not be. It isn't settled, and those who disagree with you aren't necessarily overlooking something major.
And I can't be sure what you're referring to with the parenthetical. If it's his entrance ceremony spell, we're told (and it's shown) that it's just a spell to prevent falling over for a day. If it's something else, you'll have to refresh me; my recall of hundreds of chapters/multiple years earlier in the story is not actually encyclopedic.