I don't think it's bull!@#$. In fact, it's easier to measure than fame, honour, prestige, loyalty, morale, etc. etc. I'm sure political power means many things to many people, but if it's measured by your ability to make other people do what you want them to do, then certainly getting people to follow you, swear loyalty to you, vote for you, and appoint you to powerful offices, are all evidence of political power. No metric is perfect, maybe there are other IG hallmarks of power. If so, they should be factored in too.
Its evidence, but not actually meaning there is political power. Just like there is evidence in a case, but it doesn't mean the person is guilty. Political power is the power to influence politics, which means that high-level positions it is easier to do generally, but does not mean that ruler always actually has the most political power or that a knight can't be powerful politically either.
How many vassals they have doesn't give them political power. One of my characters is very often on opposite sides on issues so the fact that I am his vassal does not add to his political power. Another character of mine is only a knight albeit an ambassador as well, and has a pretty decent amount of political power. More vassals /= more political power. For some it will, because they have very good relationships with their vassal, but it doesn't apply to everyone.
