What is the meaning of "longer electrical length = more wavelengths"?
While the document talks about more wavelengths, usually the threshold where large impedance mismatches start to matter is regarded as 1/10th of a wavelength for digital systems. Below that the practical effects of impedance mismatches are small, and at 1/4th wavelength they are already quite large. For analog and RF electronics the threshold can be even lower.
Let's first take a look at what happens in a long cable with a severe impedance mismatch at the far end:

The transmitted wave reflects at the impedance mismatch, and the reflected wave adds up with the transmitted wave. Depending on exact length of the cable, the location and the size of the mismatch, the waves add up to either a slightly distorted signal or to a full standing wave behavior I've drawn here.
With careful design and control of the cable length, the reflection can be taken advantage of, such as in quarter wave impedance transformer. However, normally we want systems to work the same way independent of the exact length of the cable. To achieve this, proper termination must be used to avoid distortion due to reflections.
But, what happens in a short cable?

Because of the short length, the phase of the reflected wave is almost the same as the transmitted wave. Thus overall distortion remains low, and there is little effect from the impedance mismatch.
The document also mentions intersymbol interference. This applies to the data transmitted, and relates to how many following bits can the reflection from a single bit disturb. However, I would assume that in most protocols it makes little difference whether the noise is from the previous bit, or a bit that was 5 bits ago.