Thats because hdparm was installed -as a dependency-
So according to the system the only reason you want it is for TLP
(and because pamac by default selects other packages … which is a debatable decision)
But yes, marking hdparm as explicit should no longer pull it along with TLP.
If it does, then use pacman (after marking hdparm explicitly installed, however you like):
sudo pacman -Rns tlp