We've used Young Again No Carb for over a year.
The label and instruction sheet for diabetic pets that you are referring to are relatively new; the food only started coming with these lately.
I don't think the warnings are "marketing hype". They're intended, I think, for people who may be switching from a typical high carb dry food like Purina or Iams.
Vets don't always know to warn about the link between carbs and glucose levels and not everyone knows to expect a glucose drop when going from high to low carbs. If you weren't home testing, the cat could be in danger fairly quickly.
I'm actually thrilled to see the words "testing your cats blood glucose level before each injection" on a pet food label!
So few people (and vets) seem to know about home testing...
What I don't like about the new labels are all of the "MUSTS".
MUST free feed, MUST not feed any other foods, etc.
We feed YA as a snack once or twice a day, but also feed low-carb wet foods. Our three cats adore YA. But, in addition to our diabetic cat, we have a cat with a liver problem who must stay hydrated but isn't a natural water drinker.
We couldn't possibly consider switching her over to a dry diet; she needs to get as much moisture from her food as she can. The YA is given with a spoonful of water on top or used in the timer feeders to make sure mealtime happens if we can't be home at the usual time.
Used the way we feed, at least, the YA has not been a "miracle cure", and I doubt that it would be even if it were the only food we fed. Like all very low-carb diets, though, it helps regulate blood sugar. Daisy is down from 2 units of Lantus, twice daily, to a scant unit up 1-2 times most days. We test before each meal and she is low enough to skip shots almost as often as not, sometimes going several days in a row with low numbers. We keep hoping she'll go all the way OTJ, but she is a yo-yo cat: numbers in the 300s one day and double digits the next, with no food pattern to be found. She's a very old kitty, and it is my understanding that younger cats are more likely to go into - and stay in - remission.
If you go on 100% YA, please post to let us know how it goes. I'm curious, but predict that results will probably comparable to the very-low carb wet foods.
As a side note, which Merrick foods are you feeding? We used to love Merrick's "Before Grains" line, but I think they've been discontinued. The Merrick "Perfect Bistro" foods are grain free, but all over the boards in terms of carbs. Daisy does well on the Duck pate, but some of the other varieties are way to "carby" for her.