We added this check to catch test cases that passes tenantID as `kTenantIdPrefix + "_0"` (which caused access blocker malfunction) . As part of PM-2346 (Native Tenant Data Isolation support), we are changing the tenant migration/shard split related code (SERVER-71179, SERVER-71184) to use tenantID as type "tenant_id" rather than "std::string". So, we no longer need to worry about the Cloud/test doing a foul play while sending the tenantID to the server.