Actually, it still uses ≤ 3 vs. = 4 arguments to distinguish between request callbacks and error callbacks. Check out the added lines to lib/router/layer.js in the commit you mention, or the equivalent functions in the current router v2.0.0 package [0].
[0] https://github.com/pillarjs/router/blob/2e7fb67ad1b0c1cd2d9e...