Hi Folks, the "lizard monster" category of concept art was just a wide ranging category to cover all the fish-like, lizard-like, frog-like, snake-like etc. humanoids and variations of those. Sorry for any confusion.
The kuo-toa and sahuagin are indeed fish and not lizards (thus a few doors back, to the side and forward), but describing the Yuan-ti as lizard monsters would be broadly correct. Serpentes is a "subsidiary branch" of Squamata, where lizards sit within Reptilia.