There is no convention wrt case for Emacs-Lisp function and variable names. (There can be any number of such "conventions" that anyone could come up with, but there are none that are followed generally.)
The "Emacs Lisp Style Guide" that you reference is just one person's proposed guidelines.
A (very) few longstanding libraries distributed with Emacs do use two styles that you mentioned: Some names are title-case (capitalized) and some are lowercase.
In some cases, at least originally, a distinction was made between commands and customizable variables intended for users and non-interactive functions and variables intended more for Lisp code. But this has not generally been respected or implemented consistently.
Generally, it makes sense to use all lowercase, especially for names that a user will type (e.g., to
M-x, as input). It's just easier that way. And in practice most names are in fact lowercase.
Use whatever you want. Use whatever you feel is most convenient for your use cases.