Except Titan likely has more water on it than Earth. Therefore unless we’re a fluke of a solar system planetary bodies with water on them should be extremely common.
Titan is outside the frost line. There’s no question that there’s a huge amount of water in solar systems, the question is if there’s a consistent transport system (comets, in this case) that moves it inside the frost line to where liquid water can, given an atmosphere and gravity, exist in conditions that match our familiar conditions for life.
The article mentions that the inner planets were initially too hot to retain water, but presumably Titan didn’t have that problem, being much farther from the sun.
Titan is interesting, but much further away from the sun, so different conditions. We want earthlike conditions, life that can sustain on anything else, is just hypothesis so far.
Mars is further out in the solar system, and I'm assuming it was further out than Theia when the collision occurred.
The article doesn't say no planets can have water, but just that originally Earth was too close to the Sun to have liquid water. Theia, according to this hypothesis, was not.
