Q: You mentioned that most of the jitter is due to high
frequency noise generated by the VCO. Since overall the PLL
acts like a low-pass filter, doesn't the high-freq. VCO
noise mostly get rejected?

A: The PLL acts like a low-pass filter with respect to
REFERENCE noise. The PLL acts like a HIGH-PASS filter
with respect to VCO noise. So, high-frequency VCO
noise almost in its entirety is seen at the PLL
output.

Why? The VCO sits at the output of the PLL. When
it jitters, that jitter is seen immediately on the
output. The only way for VCO noise to be filtered
would be if the PLL had infinite bandwidth so that
the loop could correct instantaneously.