That's because we're talking about singular functions, not types. Ti or Fi types are not going to be as socially authoritative because they're always paired with an extroverted perception function and are introverted, but speaking exclusively, Ti is more assertive than Si because it's a judging function.
None of the introverted functions are going to try and make others fall in line with their system. The extroverted functions are the ones that carry out social interaction and decision-making, hence why introverted looping will create a hermit-like antisocial individual with different hints of bitterness. Judging the introverted functions from who is most likely to believe people should fall in line with them, both Ti and Fi are up there. The order doesn't really matter because they're both introverted judging, so either Fi, Ti, Si, Ni or Ti, Fi, Si, Ni.
Although personal experience is not a reliable source, for the sake of it I will say that I have met many domineering high Ti users. Most paired with Se, but they're out there. They tend to let it show more in a workplace environment rather than social.