Yes, the Deployer + DMC can be a search head on your indexer cluster. In fact, I think it should be.
And you are right, neither the Deployer nor the DMC can be a member of the search head cluster.
But you can search an indexer cluster with a mix of independent search heads (like the DMC or Deployer) and search heads in a Search Head Cluster. It all works.
FInally, I would probably word this differently "search head cluster nodes should be on the indexer cluster for distributed search." It is a mouthful. But what I really want to say is: Any search head - clustered or not - can become a participant in an indexer cluster. The search head just needs to register itself with the cluster master and provide the secret password. The search head does not need to configure distributed search to search an indexer cluster.
HTH
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