I think some key points to know about SST:
1. The SST constructs (e.g. Table, API, Auth, etc) are thin wrappers around CDK constructs. It is not an alternative to CDK, per-se, but higher level constructs around CDK primatives that make it easier to build common pieces of infrastructure
2. You can use CDK constructs directly within an SST app. As a matter of fact, it's quite common to do so when you need functionality that SST does not natively support.