Enhancement type MOSFET - channel formation
Yes both mechanisms are involved. In my opinion Sedra and Smith is plain wrong if they claim that the channel only exists from electrons from source and drain.

That is because a MOS structure by itself (P-substrate - thin oxide - conductive polysilicon layer) so without drain/source regions, can already form a conductive channel at the edge of the P-substrate when a positive voltage is applied to the conductive layer (the gate). Like so:



When Vgb is large enough (more than Vt) a channel will form at the top of the (blue) p-type substrate. OK, it will not conduct anything to anything else but it will be there.

Perhaps in Sedra and Smith they refer to a channel of an active NMOS so where some drain current is flowing. Then indeed the electrons come from the source and travel through the channel to the drain.