You are a definitely "person who develops for web" but not necessarily a Web Developer within the context of any community which assigned a specific meaning to that title.
The definition may, of course, change over time and it already differs across communities. On the top of my head, I know many freelancers who wouldn't consider you a web developer without good Photoshop skills and many JavaScript-oriented developers wouldn't consider you one if you can't write a line of JS without using a library.
I, however, don't know any significant group which would call a person who can only author on Wix, a Web Developer. Ignoring that would surely create miscommunication and would be considered a wrong or misguided use of the title.