Importance of Source-Body Connection In MOSFET Cascode Amplifier
Wikipedia has labelled this particular diagram to distinguish it from the dual gate version. In this, a single transistor with two gates is used as a single component cascode. As there is only one body, so the effective source halfway up the channel will have a non-zero 'source'-body voltage.

It's one of those situations where what they've said is true, but not necessary. If you understand it, you don't need it. If you don't, it's confusing. You only need to delve down into the source-body voltage if you're doing very detailed circuit analysis, or doing design of the devices themselves. 

I'll think about an edit to it, but after a while, so as not to disturb the present context of this question.