You're comparing HP Penalty with old Weakness/Brink. They aren't the same status effect, and old Weakness/Brink did not function in the same way. With Weakness/Brink, your HP was slashed because your VIT was slashed, which is what I said. Perhaps it wasn't clear because I was tired, but I'll reiterate. Again.
If you want to get technical, HP is a base attribute and VIT is a main attribute. But old Weakness/Brink slashed main attributes. Hence why your HP went down - because your VIT had gone down at the same time. Same with MP and Piety. The penalty wasn't explicit to HP/MP; it was specific to your main attributes.
There's even a Reddit post from ARR talking about how Weakness reduced an individual's VIT:
This gear gives me 340 VIT / 3786 HP. With single weakness, that changes to 287 VIT / 3017 HP. . . .
. . . .Edit: Edited to add stats for when the WHM is out of PT. Solo Stats: 3655 HP / 331 VIT Single Weakness: 2930 HP / 281 VIT Double Weakness: 2205 HP / 231 VIT
Ok, I've figured out what's wrong. You're losing 15% of VIT which results in more than a 15% loss of HP. The problem is that your base VIT gives you less HP per VIT than what gear gives you. Base HP per VIT is closer to 10 (or approximately 8 if I unequip my soul crystal and become CNJ instead of WHM). So the description is wrong as I thought, but it would be correct if HP were changed to VIT in the description.
I literally gave it to you - "All main attributes are reduced by 15%/30%", for Weakness/Brink of Death, respectfully. That included VIT/STR/DEX/INT/MND/PIE back in HW because those were your main attributes back then. Now it explicitly only says "Reduces Strength, Dexterity, Intelligence, and Mind by 15%/30%", excluding VIT (which is still considered a main attribute) and Piety is no longer considered a main attribute as of 4.0 so it is unaffected, and not listed with the others.
All that information is in the Patch 4.0 Notes that I linked in my previous post. You can also use the Wayback Machine to archive the FFXIV GamerEscape page on the status effect back to any time before SB dropped and see a similar description.
Basic research goes a long way.
But there is a mathematical difference. That's what you're failing to realize. Just because you don't see it doesn't mean it's not there.
The "gimping yourself" comment was made by you well after the comment you quoted from me, along with how people supposedly don't care about a 5% damage loss (DRG-less BRDs and MCHs would like a word with you about that). But raiders do, and we're discussing raiders; not non-raiders. Context matters, whether you want it to or not.