They were wrong, it IS unlockable, only the requirements are well... kinda unique.
See, the developers thought that the X-49 was so special, that to actually fly it the player would need to invest more than time and patience. They wanted you to, in a way, BUY it! (think archaic DLC)
- purchase Direct Audio
- insert AppenDisc into PlayStation or disk drive.
- save SPECIAL DATA SAVE.
The Direct Audio is definitely worth the investment. It's not just .wav files copied to new CDs but an all-around kick-ass package. A full article detailing the Direct Audio will follow shortly.
It's also the last AC３ item that is still available at your major Japanese online retailer of choice. Brand new and at 2,800 yen, it is relatively cheap for a Japanese import. I wholeheartedly recommend it!