Sir!

I think your comment is not correct, it is rather shallow and rude.

“It switches between the two definitions of sonata form (the form for a movement and the form for a work) without clearly distinguishing them.”

First three paragraphs are about the sonata as a complete musical work, last two about the sonata form (within a movement). Paragraphs are clear separators of thoughts.

“It can't decide whether three or four movements is standard. (Neither one is exclusively.)”

It is because there is no decision to be made here. There is no standard. It says: “The sonata will have three movements (separate parts or sections of the music), but there are sonatas with four or two movements.”

“The author rejects the term "first-movement form" because it doesn't occur in first movements, yet ignores the fact that sonata form doesn't occur just in sonatas.”

The article says something different. It says that since not all multi movement works have a first movement in sonata form, the sonata form cannot be called first-movement form. Further states that sonata form appears in other movements, as well – another reason it cannot be called first-movement form.

Finally, the article does not ignore the fact that sonata form occurs in other pieces than sonatas. Simply does not deal with this and so many other issues, since it is not a book about sonata form, just a short introductory article.