apparently negate the need for a separate life jacket.
I would not count on that. For starters, I can't find that claim anywhere on their website, and that doesn't surprise me.
A life jacket, when inflated, has most of the buoyancy on your chest, and as a collar around your neck. So if you become unconscious, your head is kept out of the water and you don't drown from water ingestion. This is one of the most important design criteria to get CE approval as a "life jacket".
If the buoyancy is spread around your whole body, you may well end up in a stable position with your face in the water.
They cost less than an immersion suit.
My immersion suit (actually a dinghy sailing dry suit) was 150 euros 2nd hand, 270 new. So the price difference is minor, but I sure hope my immersion suit will help me survive in water temperatures considerably lower than +15C.
(Actually I hope never to end up in that situation, but you get my point...)