I don't think that the authors use the word "arbitrary" in the sense of "random" or "one way". This is precluded by the word "scheme" earlier in the same quoted sentence.
By "arbitrary" they mean either that their method is invariant to the exact scheme of anagramming employed in the VMS (see the context in which the word "arbitrary" is used on p.75 above), or that the rule of anagramming may shift from vord to vord (i.e. be different for different vords). It's difficult to say what of the two things they actually mean, but for sure they are not arguing one-way cipher.
Some combination of abjad and anagramming is a tempting path, of course, because it potentially could explain both for low entropy and strange patterns of glyph positioning.
By "arbitrary" they mean either that their method is invariant to the exact scheme of anagramming employed in the VMS (see the context in which the word "arbitrary" is used on p.75 above), or that the rule of anagramming may shift from vord to vord (i.e. be different for different vords). It's difficult to say what of the two things they actually mean, but for sure they are not arguing one-way cipher.
Some combination of abjad and anagramming is a tempting path, of course, because it potentially could explain both for low entropy and strange patterns of glyph positioning.
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