Do I Have to Deploy a Cluster to Use Shared Disks?

Yes.

If you simply attach a shared disk to multiple servers, files cannot be shared among them. Because there are no mutually agreed data read/write rules among servers, read and write operations from them may interfere with each other, or unpredictable errors may occur.

Shared EVS disks do not have cluster management capabilities. You need to build a clustered system for data sharing, such as Windows MSCS, Veritas VCS, and Veritas CFS clusters.