There is a post from last year on the second page of the linked topic, stating that removing DVI decreases the problem, but does not solves it completely.
Anyway, asked Dezz about it, and he said he wasn't aware of such a problem and that the SJ XAGA normally doesn't send analog and digital signals at the same time (it checks instead if an analog or digital connection has made and sends signals accordingly). His decision to provide means for shielded/twisted pair cables is independent of it and based on experience with earlier products, like the SJ XAGA Lite, where he had to provide shielded cables to prevent picking up noise from the motherboard (perhaps it is more prevalent in case of the A1200 as there are higher frequency signals in it than in an A500, just guessing), as well as air. There were also rippling a little with ribbon cable. The UTP/STP cable compares to a high quality monitor cable. Also he said DVI's digital differential signals are meant to be transferred on twisted pairs and it can affect high bandwidth signals.
It is perhaps not neccessary, but beneficial, I assume.
We have had all kinds of expansion boards sitting on chips in Amigas since the early 90's when the A1200 came out with these SMD chips on board. There are signals impossible to access otherways. Some of it not even in an A4000.