Before remediation is attempted via Update Manager at cluster level, few pre-checks are performed. In the case of DRS-disabled cluster, Update Manager runs DRS check on each host regardless of whether VMs are running on the hosts and then throws an alert about DRS enablement:
"Pre-check found problems what will block remediation. See details below. The blocking problems may have caused the pre-check to complete prematurely. Therefore, after fixing the problems, please re-run this pre-check"