I found this too. The official statement from this product saying they've added support for Samsung SGH-T399 is posted in a GSM forum
HERE. The OP has some youtube videos showing the software and unlocking the phone's IMEI and stuff.
However, there is a note: Before performing "Direct Unlock" ("Repair IMEI", "Repair Network", "Read Security", "Write Security", "Read EFS", "Write EFS") operation for SGH-T399, you need to root the phone manually or write special firmware "SGH-T399_for_Unlock_and_IMEI_Repair" from Support Area.
Plus, one of the first replies to the tread was a user who tried it on the T399. I joined the site and asked this user who had successfully unlocked his phone using this software if this was legit. Turns out he managed to unlock the phone, but has not tried root. He suggested for me to try flashing the support file which has root incorporated. I told him to send me the flash file, if he can. We'll see what happens. That flash file might have what we need to get in. It might not work because their files come in proprietary extensions (Octopus – OCT files for Octopus Box).