I was under the impression that DMCA notices were for the removal of infringing content, not alleged anti-circumvention tools. That's what the law seems to specify. The notification and takedown process is specified for infringement of copyrighted works, not distribution of anti-circumvention tools. EFF's explainer video seemed to concur with the assessment that DMCA notices are not appropriate for 1201 violations, only for removal of infringing content.
1201 enforcement appears to be through other mechanisms, such as criminal liability and statutory damages. Presumably those would require something more than a letter or notice claiming violation.