I don't believe this is correct.. otherwise, it would be a major issue for a lot of people who've soft bricked. it does have update protection for the bd. instead of searching for PS3_UPDATE, it searches for NOT_UPDATE for bd only. I searched for it in the emer_init.self (the recovery menu), and I don't believe I saw any changes to usb updating.
edit: I just double-checked using 4.81.2 as a base. nothing has changed with usb updates, only bd or bdvd have been changed to NOT_UPDATE and NOT/UPDATE respectively.
I worked it out - it was bad hdd. I bought new one and installed it. And I was right - you can't install rebug from recovery, it gives you corrupted file error every time. But I flashed ferrox 4.82.1 and then rebug from OS. Installed psnpatch and everything working fine.