When you say that full defense spec has lower single threat generation, how much lower? Enough to not be able to main tank a boss? Or is possible to MT with that spec?
What he probably means that threat from damage is not as good, but that's not how tanking is generally done in SWTOR. You generate threat by doing your high threat moves, then taunting. Most threat generation is done via taunts. Taunting when you already have threat only increases your threat. High threat moves have a threat multiplier and do not rely solely on damage. If your taunts are constantly on cooldown, you are unlikely to lose threat.
Many main tanks are full defensive, and it certainly makes sense when doing difficult content. It's probably easier, in fact, because it was designed for tanking and includes some high threat generation moves. In general, hybrid specs can be made to work, but they frequently require more care.