Totally agree. It doesn't help at all. Pseudocode would have benn sufficient. Fake machine code? Nope.
OTOH, pseudocode is easier to understand, partially because you don't have to think about some of those extra things that don't actually matter in the cost model. For an expository text (and, TACOP is an expository text), I think MMIX comes down on the wrong side of the tradeoffs.