basically........... (as there was a previous thread and I remember typing a paragraph or 10 :lol
the entire thing is just a marketing ploy, same as "grain free for rabbits" or "beneficial fibre" listed on packaging of rabbit food. there isn't any real need for a indoor rabbit. yes they're not as much outside as outdoor rabbit therefore you would think they lack in vitamin D, but they don't. if you're feeding a well balanced diet, handful of veggies, some dried forage, good quality hay then they'll be just as healthy as the outdoor. the pellet (providing it is high quality) will provide everything they already need so anything on top is more just an added bonus.
the price is hefty too for that 1.5kg bag (yikes) so I just wouldn't bother. for that price you can get 2kg of their standard food, but also as they sell the standard in bulk you can get more for less once again.
tl;dr: its not really necessary and is probably just catching onto the market (grain free for dogs and cats, indoor for indoor cats) without much consideration. otherwise, before they came out with this we would be seeing a lot of cases with rabbits who are vitamin D deficient...which wasn't the case.
I wouldn't change your buns current food to it. its it bad nutritionally? no. but the price & the fact to me, it isn't worth it.
they'll be getting all their vitamins from good hay, their pellet ration and a handful of fresh veggies/herbs each day, and any dried forage you may or may not feed.