I don't think it's a leap at all. Janek being a ship Captain very likley has some sort of military background. Someone with military background would easily recognize a military outpost. Add to that it appears that they were stockpiling those cylinders like one would stockpile weapons or ammunition. It seems as though they weren't meant for anything good at least as far as humanity is concerned. It's also apparent that in production of these cylinders and what they contained something had gone horribly wrong. It's quite obviuos that this is not the Engineer's home planet. That it is some sort of outpost or station of dome kind. Holloway does make a comment that about it being "just another tomb"