There are several implementations that have their differences. It's kinda similar to "Lisps" in plural. Lisp is a programming language, but it's also a family of similar languages.
Also, languages like K and J, maybe Q could be considered part of APL family due to their array oriented design and single character operator syntax.
APL is the original language, and has spawned a family of similar languages, usually ones that don't use the same extremely specialized symbolic charset of the original language.
This is similar to how languages of the Lisp family are called 'Lisps'.
Exactly. You more commonly hear this when talking about Lisp. Racket is a Lisp and so is Scheme or Common Lisp, and what we mean by that is dialects.
In the article he mentions J, K and Dyalog as APLs.
