I certainly don't know, but I think what the guy is saying is that they are necessarily analog switches here because of the speed they have to operate at.
You can use an analog mux on digital signals, but it probably won’t be as good as a digital one. However, you can’t use a digital mux to switch analog signals, it won’t work.
Edit: the one exception to this is for bidirectional digital signals you may want to use an analog mux, since you don’t know which side may be driving the lines. This would apply to DRAM since the data lines are bidirectional, so probably using an analog mux.
Digital the values “round-to-nearest”, so if supply is 1 V, then 0.6 V, 0.9 V, and 1 V all mean logical 1. Whereas for analog those different values mean different things.
Also, for digital you want to push the signals to the extremes so they are easier to read, while for analog that would be destroying information, you want to preserve the value.
(In practice there are fundamental limits to how much useful information you get out of analog signals due to noise.)
Clearly the output signal has to be 0 or 1, which must mean the signal is boosted or reduced to an appropriate level (assuming we not trying to encode multiple values onto a signal; I'm just talking binary). So I guess the signal has to be captured (latched?) then modified. I don't think a clock is absolutely necessary as there have been asynchronous digital CPUs, but the presence of a clock clock is very typical as well.
A clock is not necessary in this definition. For example a simple logic AND gate is digital and has no clock. The input is not captured or latched before being modified; nevertheless it is a digital circuit.