Well, it is actually the right advice. You need to have more tags set in the editor for the individual trainers, if you need to control the text shown in the slider. This is how excerpts work in WordPress and we are just leveraging what is already there in WordPress. There is no excerpt_count attribute in the trainers_slider and hence you cannot control excerpts that way.
Also, if you have loaded the sample data, you should see the more tags populated for all of the trainers in their respective edit pages.