Transient time of a circuit and pulse width
What the statement means is that in order to have a pulse signal, the interval between edges must be comparable or larger than the duration of the transients. Otherwise the circuit won't have enough time to complete the voltage swing between the logical levels, and will be predominantly in the transient phase, resulting in meta-stability or unstability.

If you think about it, if it takes 100ms to change level and the pulse only lasts 1ms, it will be filtered out and won't result in a clear waveform.