What is the SLA for HA Cloud SQL?
On this page it says
Regional persistent disk support for Cloud SQL and the Cloud SQL HA configuration have full Service Level Agreement (SLA) coverage. An HA-configured instance is charged at double the price of a standalone instance. This price includes CPU, RAM, and storage.
On this page it states
During the Term of the agreement under which Google has agreed to provide Google Cloud Platform to Customer (as applicable, the "Agreement"), the Covered Service will provide a Monthly Uptime Percentage to Customer of at least 99.95% (the "Service Level Objective" or "SLO").
Am I correct to interpret it as Cloud SQL and Cloud SQL HA have the same SLA despite HA is double the price?
Yes, the SLAs are the same, but that not represent the real availability. It's simply the financial commitment de Google on Cloud SQL.
Don't mix your service availability, and the confidence threshold that take an operator to financially compensate you. On the other hand, do you prefer money or your user happiness?
If you can afford HA, do it, it's worth it!