The source also gives “this horse” and “this wall” as particulars.
“Horse” by itself names a general kind and may be predicated of many horses. “This horse,” in a context where the demonstrative identifies its referent, picks out one determinate horse.
“Wall”: a general concept.
“This wall”: the particular wall identified by pointing.
The referent of a demonstrative must be fixed by context. Saying only “this,” with no identifiable target, does not by itself give the hearer a determinate particular.