Data consistency in SQL AlwaysOn Availability Group
From the same SQL Shack article that you quoted in your question:
- The secondary replica also contains a redo thread, and it is independent of the log block process in SQL Server Always on. Redo threads reads the logs from log cache. There might be a delay in processing by redo thread and log records might not be available in log cache because it is already hardened to disk. In this case, redo thread read log blocks from the log disk.
Which I read to mean that the log hardening process doesn't make the changes immediately available in the secondary database but rather that the redo thread on the secondary needs to process them first.