Why does radiated emission of a PCB decrease as the frequency of the signal increases?
It is not easier to design for higher frequencies and the confusion you are experiencing is caused by the misconception of thinking that the two solutions compared in the book would be as good at 2 GHz when they are actually as bad.

In other words, traditional signal and power traces are great antennas anyway at those frequencies, but usually the signals spectrum has already faded so low that it is not a problem anymore. This can be easily understood if you think for example the envelope of the spectrum of a trapezoid which is quite common signal waveform. The amplitude decreases 20 db/dec after the frequency of \$\frac{1}{\pi \cdot T}\$, where T is the signal on-time and 40 db/dec after the frequency of \$\frac{1} {\pi \cdot t_r}\$, where \$t_r\$ is the rise time of the signal. Therefore, for example the spectrum of a signal with risetime of 10 ns has a second knee at 32 MHz whichs is almost two decades under the frequency range of interest. Therefore, the signal amplitude has faded to about 1/10000 because of this knee alone at the frequency of 2 GHz.

If we were studying discontinuties in structures designed for signals having considerable spectral content at 2 GHz, it would be clear that the circuit with discontinuties would behave worse.