Slope of VTC for FET
The figure is drawn incorrectly.  The written words and equations are accurate.  A full explanation is available here:  https://dfdeboer.github.io/errata/Sedra_Smith/7th/S_S_7_ER.HTM#P374B (I am the author of that Web page.)  Kudos to the OP for an observant eye to tie those details together!