If the diet is based on vegetables, then no, it is not very digestible. Whoever uttered this statement (actually, several veterinary doctors have uttered this statement in articles, some of which are posted on the web!) has the burden of proof on them; it is up to them to prove this statement since everything we know about raw foods, including literature discussing the digestibility of wolves' food in the wild, indicates that raw diets are VERY digestible.
This is the primary evidence: dogs fed a raw meaty bone diet produce stools that are one-third the size of the voluminous stools of kibble-fed dogs. That equates to a two-thirds size reduction in fecal output!! This indicates a higher digestibility; vets and dog owners know this very well. Smaller stools = better digestibility. Raw diets are 90-97% digestible (includes the bone!!), whereas most kibbled foods are 40-70% digestible (some of the better, holistic kibbles have higher digestibilities than this figure). This explains why most kibble-fed dogs have such huge poops: most of it is undigested grain and filler, also known as "fiber". Even premium dog kibbles, while more digestible than many other kibbles, yield stools that are rather large compared to a raw-fed animal's poop.
If someone thinks raw diets aren not very digestible, then they must answer the question of where all that meat and bone go! It is ingested in large quantities and then comes out the other end in small, odorless, well-formed stools. If it is not very digestible, then what happened to all that "undigestible" stuff?