I'm still struggling to understand how that happens. You've got a professional programmer who's just randomly writing code for imaginary requirements they made up? I mean sure, obviously that's bad practice and unnecessary, but who does that, and why do their managers and technical leaders allow it?

I've always taken YAGNI to be a cautionary principle about over-engineering and over-generalising software designs before there's sufficient expectation that the extra flexibility will justify the implementation cost. I'm not sure I've ever seen it used (or needed) in the context of someone just randomly adding arbitrary functionality because they felt like it, and it had never previously occurred to me that someone might restrict the term to only that scenario as Fowler seems to be doing in the quoted comment.