Yeah but providing an opt-out toggle for "legitimate interest" is a good indicator that the interest is actually not legitimate enough to require manual opt-out. And in practice I've seen it mostly used as a gotcha to make it harder to opt out of every single advertising partner individually.
"But we want to show you ads to finance our website and our advertising partners want to abuse your privacy" is not a legitimate interest.
This gets back to GP's point about GDPR vs ePrivacy Directive. Legitimate Interest is a separate Legal Basis under GDPR (and does in fact allow opt-out). But the ePrivacy Directive does not recognize Legitimate Interest. You cannot use Legitimate Interest as a basis for making a cookie opt-out.
"But we want to show you ads to finance our website and our advertising partners want to abuse your privacy" is not a legitimate interest.