> The explanation of the stirring of the water plays no role in the understanding of the miracle or of the rest of the chapter.

^^^ From The Angel at Bethesda, John 5:4.

Oops! Bad copy-paste of the link. Must be 60 The Angel at Bethesda, John 5:4.

So, according to that link, John 5:4 was a comment that sneaked into the text.