If I understand it correctly, the loyalty ability only affects the chance of flipping, but not the influence required to initiate a flip.
So, comparing a Korx and a Yor planet, both will start showing warnings that a "flip" is near at the same influence value (enemy influence about 4 times higher than owners influence). Each turn, there is a chance the planet will flip, it is this chance factor that is modified by "loyalty", the Yor have a strongly reduced chance to flip, while the Korx have a slightly increased chance.
However, due to the peculiarities of "random chance", it is entirely possible that the Yor planet will flip on turn 1, while the Korx planet stays Korx forever. It's just not very likely. On average, you will see Yor planets last considerably longer than Korx planets when under influence seige, but in the particulars you can never know for sure. This makes loyalty an extremely useless ability for the player, because it can never be perfectly relied on to do its job. You're better off pumping those points into population growth or influence!
Whether it scales over 100, no idea. I have no inclination to try.