As someone who has gone on long stints using beans as a primary protein source, this is simply not true IME. But it does depend on the types of beans and the cooking methods.
AIUI a major component of what determines gassiness is if there's a combination of fiber (especially soluble fiber) and starches/sugars which become locked up in that fiber so they pass through the small colon intact. This is a desirable situation from a health/weight-gain standpoint, since you've avoided absorbing those simple carbs. But it feeds the biome of the large intestine, producing gas.
Legumes tend to have those ingredients... there's no reason to expect it to stop occurring. Ex: 100g pinto beans contains 26.22g carbs of which 9g is dietary fiber, the remaining carbs is stuff your gut can likely use to make gas.
This is simply not true. I grew up in a family of multiple generations of lifelong bean eaters and the gas never goes away no matter how you cook them.
