Ventroflexion in the tolt is part of the frame in which the Icelandic Horse (and any other racking horse) carries itself to perform the gait.
There are trainers, however, who may not understand ventroflexion and are instructing their students that the horse is round. In fact, the same is mentioned on at least one video of Icelandic Horses. It's good for the horse to be round in walk, trot, and canter, but hollow (ventroflexion) is part of the tolt (racking) gait.
Somewhere along the line between Iceland and Germany, dressage terms must have been applied to Icelandic Horses. Possibly the terms were known to be different but used anyway, or possibly the terms were used unknowingly.
Germany (and Europe) are relatively new to gaited horses, thereby being somewhat behind the knowledge of American gaited horse judges, trainers, breeders, and equine bio-mechanics experts.
With the Icelandic Horse, we have mostly new (novice) horse owners / breeders who were looking to the Icelanders and Germans for riding and training methods. A cycle started: new horse owners taking lessons or clinics from trainers passing on incorrect information. There was no one knowledgeable enough with an interest in Icelandic Horses to question anything, so the "experts" were born due to the novice horses owners placing them on their pedestals; sort of a co-dependent relationship.
In any case, it's not logical to perpetuate the misinformation. There's no time like the present to start using appropriate terminology! The benefit of this, is that everyone will have a common language.
For the most part, Icelanders and Germans have "gotten" tolt from their horses by taking the "dashboard" seat (la brida). Their riding style included high hands, heavy contact, leaning back behind the action, sitting on the horse's loins, and even going so far as to manufacture saddles with extended panels to dig into the horse's loins to get tolt.
Lots of horses (or other equines), even non-gaited, will tolt / rack with this type of riding.
If a horse, tho, is naturally gaited, these methods are not necessary. A little finesse and better training methods are needed.
Back to ventroflexion: As noted above, ventroflexion (hollowing) of the back is something that the horse HAS to do to be able to tolt. It is seen in all pictures and videos of Icelandic Horses.
Take a look at any Icelandic Horse magazine, book, website, or video. Take a look at Icelandic Horse logos: they show the horse in ventroflexion. For example, the logo of the United States Icelandic Horse Congress shows the horse with a hollow back, high croup. Look at breeder's logos... how does the back look -- flat, concave, convex?. Look at the pictures of Icelandic Horses in the Gaited Horse Magazine. Check out the logo of Tolt News. Those involved with Icelandic Horses should know this.
And it's OK.
The thing that's not OK is for trainers to not know this!