I'm confused, if he had not been arrested yet and came in voluntarily - what's the problem? The police don't even have to "read him his rights" until is being arrested. Anything he voluntarily says or doesn't say before arrest is fair game (which is why lawyers generally want you to shut your mouth until they get there).
I haven't read the opinions in that case but did just review the link. My reaction is that the author is over-blowing it a smidgen. All this case seems to say is that whether you answer questions, invoke your rights appropriately or not, prosecutors can testify about your demeanor, the circumstances, whether you were calm, sweating, nervous, fidgetity, etc.
It makes me nervous though, because there are no corroborating witness except you, so if the po-po gets up there and tells an "improved" version of what occurred, you are faced with having to take the stand to refute it, and you may not want to do that for very sound reasons having nothing to do with the ultimate conclusion of guilt or innocence. Moreover, you may not be believed. I would exclude such observations as more prejudicial than probative.
There are those out there who believe that police are saints still in their earthly toil, whose powers of observation, recollection and veracity are flawless, and can be trusted and believed whatever they say under oath, but don't you believe it. Sometimes they do all that, God bless 'em, but other times, when need be, there is a temptation to recall the facts as they wished they had happened, no tie goes to the runner etc., or as we sometimes crudely put it, like golfers they "improve their lie." I've caught a couple of them at it, in open court. Since my criminal trial experience is very limited, having two instances is statistically very bad, and may not be enough of a sample, but nevertheless, like seeing your wife riding on the back of a motorcycle driven by a sailor, you can never trust again afterwards.
Luckily, I have enough willpower to control the driving ambition that rages within me.