No urban legend, many many documented studies have been done including ones by LED manufactorers , here's just one example
https://californialightworks.com/uvb-light-and-thc-potency/
" Growers have realized an increase of up to 30% THC percentage by using supplemental UVB light but the actual mechanism was not well understood."
I recommend an agromax pure uv t5 bulb.
AgroMax Pure UV T5
The AgroMax Pure UV T5 provides your garden with a concentrated blast of UV-A and UV-B spectrum light. Indoor plants, by virtue of being grown under artificial light, have little to no exposure to UV in traditional setups, however, UV light is a powerful bloom boosting light of which many gardeners have yet to take advantage. Scientific studies show that when plants are subject to UV-B light many of their natural defense mechanisms are triggered. Think of it as your plants producing their own sunblock to resist the UV rays. It is exactly this defense reaction which is targeted by many indoor growers. In many plant species, exposure to UV-B light will significantly boost and cause changes in phenolic compounds, flavonoid pigments, resins, oils, etc.
Gardening with UV Light
UV light is excellent for increasing yield and bettering the overall quality of your plants, but one has to be cautious not to overexpose their plants. Overexposure to UV light will not have beneficial effects on plants, too much UV light can "sunburn" your plants. When treating your garden with UV light it is always best started with a low dose amount, with the lamp placed further away and with a shorter "on" time duration. Proper protective eyewear and long-sleeve clothing are necessary when working with UV lamps. Different species of plants have different innate tolerances for UV-B light, dependent on where that particular species is indigenous.
Or use a lighting system that has a proper spectrum.