I can't see anything in the linked article suggesting that other 50x responses will result in poorer search engine performance - I would expect that any of the transient-fault 50x responses would have the exact same search engine impact. The important part is not returning 200 OK and having googlebot index your blackout page.
It just so happens that 503 is the semantically correct response for this situation, while other 50x responses are not.
I'd really rather everyone use the correct status codes (503 indeed being the correct one here) than try figure out what their SEO impact may or may not be. I'm quite sure Google understands HTTP.
It just so happens that 503 is the semantically correct response for this situation, while other 50x responses are not.