My Hipad Pro with no OTA update still have L1 certificate intact as shown in the screenshots below:
I have not flashed the firmware from Anathema post yet. But I assumed that this firmware would probably unlock bootloader of Hipad Pro just like your claimed Hipad Pro got unlocked. This firmware is probably the latest updated available with 2021.08.05_02. I extracted the archived firmware and checked some .img files which are valid for Hipad Pro. So I assumed this 2021.08.05_02 firmware is valid for Hipad Pro.
BTW, can you checked your bootloader is still unlocked after OTA updated?