Hrm fair point. I'm not sure DNT could have been repurposed to imply consent under much newer regulations, but you're generally right, this mechanism predated the EU regs and somehow was passed up.
"Somehow" is because there was nobody enforcing it, so nobody had any incentive to honor the request. Legislative approach is the only way to have an actual effect.
DNT was also intended to be an explicit opt out. However Internet Explorer enabled by default for three years, giving the industry an excuse to question its validity and ignore it. Privacy centric Microsoft or intentional sabotage?