Why is the worst case input/output impedance of an $RC$ filter $R$?

I’m going through the Art of Electronics in my spare time, and I ran across a problem I’ve been scratching my head about for a few days. The book states that in the worst case for any $RC$ filter, the input or output impedance is $R$. I initially thought this was in reference to the point at which power first hits the circuit and there’s zero charge built up in the cap, but then I remembered that all caps have an initial reactance, plus the book also says that the input signal sees a load equal to $R$ at signal frequencies so that can’t be why. I found this answer, but I’m having a hard time understanding the intuition behind the calculations. The second step examines Xc as it approaches infinity, so it looks like I need to take the limit of some function, but I don’t understand how they went from line 2 to line 3.