Applying manifests in Azure DevOps pipeline
Once again, come across the answer right after asking the question.

Needed to look at the Kubectl task and not the Kubernetes manifest task. Particularly the Commands section:

- task: Kubernetes@1
  displayName: kubectl apply using arguments
  inputs:
    connectionType: Azure Resource Manager
    azureSubscriptionEndpoint: $(azureSubscriptionEndpoint)
    azureResourceGroup: $(azureResourceGroup)
    kubernetesCluster: $(kubernetesCluster)
    command: apply
    arguments: -f mhc-aks.yaml