Does titanic hydra also do damage to the main unit hit or only to the surrounding units? I'm not sure what splash damage mean.
It also deal damage to the primary target
As per the teamfight tactics wiki for Tytanic Hydra :
Basic attacks deal 10% of your maximum health bonus damage to the target and all adjacent enemies.
And to adress XtremBaumer's answer, in the basic League of Legends game, the Tytanic Hydra deal bonus damage to target on hit, even if this effect may be considered faintly separated from the cone aoe.