Unless the someone invented state-of-the-art anti hacking hardware and couldn't think of any better use of it then protecting the secrecy of hdd firmware, then that is just a lie (or knowledge gained by reverse engineering the firmware is not considered "public" information). If you are interested, here is a story of someone modifing hdd firmware [1].

[1] http://spritesmods.com/?art=hddhack

I guess writing a new hard drive firmware from scratch, without inside knowledge, would be close to impossible.

But why start from scratch if you can just modify the existing firmware? And that seems to be perfectly possible: http://spritesmods.com/?art=hddhack

I'd say the most difficult and resource consuming part is to make versions which work on as many brands, models and revisions as possible, and make them all robust enough so they won't be detected because of random malfunctions.

But I don't know anything about hard disk firmwares: Perhaps they are not too diverse and once you know how to modify one drive, the others will follow easily?

Hard disk firmwares are already under attack from anyone who develops tools for data recovery.

I have personally added new functionality to binary libraries on wintel, and in embedded firmware for instance communications devices. Just jump somewhere else and jump back where you came from, there is often nothing preventing you from doing that.

There's a discussion of this over in the other thread. The general consensus there is that this capability is not that remarkable, and individual hackers have done similar things on a smaller scale.