It only means what people think the word describes. There are some who seem to equate lots of detail and transparency with a bright sound. But many don’t. There’s no definitive answer.
Some people say a highly detailed system is a little bright. Think of the opposite, a warm or lush sound. Wouldn’t that at least imply a loss of detail? So it’s just words that people assign to their own understanding of sounds. Some words have almost universal agreement, some don’t.