A hot standby is not a read replica. It's a set of servers running in a different Availability zone mirroring current prod, that is configured to automatically fail over to if the primary is offline. It's been a few years since I personally set this up in AWS, but at the time, those servers were completely unavailable to me, and basically doubled the cost of my production servers.

The fact that a hot standby is usually in some sort of read-replica state prior to failing over is a technical detail that AWS sort of tries to abstract away I think.