Update. OK, so I unlocked that Demiurge truck via a cyber engine tweaks console. Along with all other vehicles in the game. Purely out of spite and because couldn't unlock Demiurge separately. Hope some quests will not get broken cause of this.
So there's big mystery FF:06:B5 and if you follow all the steps of that hidden quest you'll get that monster truck. So. Is there any mod or console command to spawn that car instantly? I've been trying to do this the "right" way but F that noise.