Probably the composer just overlooked the non-obvious defences. After all, it was 1976, and the composition is very lighthearted.
Here's a different one, obviously a ton of cooks but it's the concept that counts. Like the previous one, I got it from Paul Lamford's 100 Chess Puzzles, attributed to T.G. Whitworth in 1987, but it may have come from his book of [Hermanis] Mattison's Chess Endgame Studies published that year.
How good are you at mating with bishops? This is not a complete joke, but certainly not your average chess position either.
#16: I'm trying to figure out whether there's another solution. :P
Comp says 0 solutions. :O
Maybe the author knew that, however, and was poking fun at how humans will see obvious patterns (like the Kings consuming lines of Bishops) and miss defenses outside the pattern (like 1...Bg5).