Yes it is. The fact that the more you eat beans, the faster it digests, is not a myth.
Try adding them into your diet in 1/4 cup increments, and increasing very slowly.
Although, there are other ways to increase the digestion rate of beans, but as for your question, it's certainly not a myth.
What Makes Them So Hard to Digest?
Besides the phytic acid contained in legumes, the harder beans such as kidney and navy beans contain oligosaccharides. This complex sugar is impossible to digest without some help because humans do not produce the enzyme alpha-galactosidase needed to properly break it down.