I never said hardware region locked. I said digital region locked.
Even you can install and play Physical region 1 disc on region 2 account.
And the region lock is not just the DLC. It's for all digital content. Including full games.
That's why I said digital region lock.
I think you got confused with PS4 set as "primary". Because setting ps4 as primary ties your license to the hardware. Thus allowing you to play any digital games and any dlc from any regions.
PS hardware is not region locked.
AFAICT the only things that are locked are;
1) if you buy DLC you have to ensure it's from the same region
2) if you buy a game from one country you can't download it from another
Personally I would not describe this as "PS4 digital content is region locked."