It should "sufficiently" shuffle your lines.
However, if you're pedantic about the degree of "randomness", you have to send it through the alias several times, due to the way it works.
Actually even with three iterations you're not going to achieve reasonable randomness. And this is only with a set of 5 lines. It would scale terribly as the number of lines increased. It's just a flawed method to begin with (although at first look I thought it might work, too).
The only really valid method would be to manually randomize the data without the use of /filter. And since this option exists, there's really no point in adding this functionality to /filter.
For example, if you want to randomize lines from @input and put them in @output, you can just do something simple like this:
while ($line(@input,0) > 0) {
var %i = $rand(1,$ifmatch)
aline @output $line(@input,%i)
dline @input %i
}
which would only require one iteration.
