Waterfall lives up to its name: once you're over the edge, there is no way but down. The smallest change in requirements; or misinterpretation of those requirements; or miscalculation regarding scale, speed, or technical feasibility; and the end result is a dog of a system that condemns your users to perpetual pain; your programmers to constant make-do; or the project to cancellation. Or all three.
I see no aplicability for scale, speed or technical feasibility here since this project is not meant to be fast at this moment. Moritz mentioned earlier that Rakudo is 10^2 to 10^3 times slower than Perl5.
But for anything that involves leading edge development, or more than one man can keep in his head at one time, waterfall is useless. And the cause of many billions of wasted £/$/¥ over the past 40 years.
Obviously there's no concern for money spent because this is a volunteer project as Moritz mentioned and Chromatic also mentioned this. And time is also not a problem since as they both mentioned they want to build something cool that has never been attempted before. So it doesn't matter how long this takes. Maybe forever, maybe never, maybe in 20 years. They want to build something that has never been attempted, it's like talking about the atomic bomb
(except that at Los Alamos they trained hardcore physicists that were inventors, and they really knew what the hell they were doing and the government invested huge amounts of money in the project)