It often does colloquially refer to schizophrenia, I've heard people use the word this way for as long as I can remember. General purpose English dictionaries document this colloquial meaning, I checked several to be sure:
"(archaic) schizophrenic" https://en.wiktionary.org/wiki/schizoid
"characterized by, resulting from, tending toward, or suggestive of schizophrenia" https://www.merriam-webster.com/dictionary/schizoid
"suffering from or behaving as if suffering from schizophrenia" https://dictionary.cambridge.org/dictionary/english/schizoid
"Informal. of or relating to schizophrenia or to multiple personality." https://www.dictionary.com/browse/schizoid
"Someone who is schizoid has schizophrenia" https://www.collinsdictionary.com/dictionary/english/schizoi...
1. It is their blog, and they probably assume their audience is educated in what schizoid means or is capable of looking it up.
2. Schizoid does not colloquially refer to schizophrenia, and schizophrenia is not a pejorative for "crazy," these are both real and distinct (but related) disorders that do not require anyone talking about them to dichotomize them from what you might assume people misconstrue their false definitions as.
Also, it's not an "extreme form of introversion" - it's much more than that, and putting it that way minimizes the serious nature of the disorder.