Again, this is not necessarily true. Many, many people successfully keep neons with angels. Some for many years. What I would say, is that if you attempt it, 1. don't attempt it in a 10 gallon tank, and 2. be prepared that some or all could potentially get eaten. But, if the neons are big enough, and the angels are small enough, they often will do fine together for many years, even after the angels have grown large enough to fit the neons in their mouth.
Here is an example of someone who has kept angels and neons together for decades, and reports no issues:
Keeping Angelfish (Pterophyllum scalare)