Loading effects of a signal source
Consider two cases:

a) \$R_L\$ = 0 (zero)

and 

b) \$R_L\$ = infinite

It is obvious that a) will result in \$V_{out}\$ = 0 as the output is shorted, so this is a useless situation.

It should also be obvious that b) will result in the desired \$V_{out}\$ as there is no load on the output.

So the conclusion is that \$R_L\$ should be as large as possible.

Now we can get to the \$Z_{in}\$, \$Z_{out}\$ discussion. There you have to be vary careful what you call \$Z_{in}\$ and what you call \$Z_{out}\$!

The book uses:

\$Z_{out}\$ = the output impedance of the voltage divider.

\$Z_{in}\$ = the input impedance of the load = the load impedance.

You confuse yourself by mentioning the emitter follower.

For any amplifier there is a \$Z_{in}\$ and a \$Z_{out}\$

Now we connect the amplifier's input to the voltage divider's output.

Note that then the voltage divider's \$Z_{out}\$ needs to be smaller than the amplifier's \$Z_{in}\$.

Remember that \$Z_{in}\$, \$Z_{out}\$ are usually referred from the viewpoint of the circuit you're discussing. So one circuit's \$Z_{out}\$ connects to the next circuit's \$Z_{in}\$.