Not exactly true. You can load on unofficial FW's but you'll lose compatibility with their own FW's, meaning you'll have to stick to PSFreedom etc. I have two PS3Keys, so i will be doing that to one of them, as they are taking their sweet time to create a new kernel with Hermes.
Original PS3 + Slim PS3 - 3.41OFW - 640/320GB - 3 DualShock 3 Controllers - PS3 Remote - PS3 Keypad - x2 Move Controllers/1 Navigation - Jailbroken - x3 PS3Key v4.0 - Maximus AVR USB.
2 Fat PSP's - PSP1000 - OFW 5.00 - PSP1003 - CFW 5.00 M33-6
Original PS2 - Chipped
2 PS1's - Chipped