He means team synergy, where Olaf brings nothing to the table in terms of team bonuses. So the berserker and glacial power ups go completely unused.
And he's not wrong, either. Of all the 4 cost units, the only one I'd consider using by itself is Olaf, maybe Janna if I can pick up another Mystic or if I have a good enough frontline. Like, Brand's fantastic as a unit, but without the mage or infernal buffs, his damage output can be safely gimped with stuns or assassins. Olaf meanwhile is absurd just by himself even without a complete stack - so long as he has guinsoo's he can do some asinine shit.
And my god if he actually does utilize the glacial buff, you might as well just write off that game unless you've hit level 9 with mages and a 4x warden buff.