Here's the most recent study I could find that investigated this effect in humans, and it seems to indicate that the juice (not an extract) has some benefits.

https://www.ncbi.nlm.nih.gov/pubmed/29021286

On the other hand there have been some mouse trials on an extract of the seeds of the fruit (NOT the juice)that have been promising, but mice are very much not people as scientists will tell you.

So, some reasons for hope but nothing definitive yet.

More broadly though, fruit is generally actually really good for diabetics, as the sugars are released slowly into your bloodstream, so I don't see that this advice will necessary be of any real harm.

https://onlinelibrary.wiley.com/doi/abs/10.1002/ptr.740

(that's the study on pom seed extract's effect on diabetic rats)