How actually rds parameter group is updated with multiaz failover reboot

Based on this document https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_RebootInstance.html

Trying to exactly understand what actually happens like the basic case below:

1. There is a none microsoft mysql rds instance, with multiaz enabled and default parameter group
2. Then change the default parameter group of the instance to the new custom parameter group, with pending reboot
3. Then manually reboot the instance, in order to apply the new parameter group

According to the document, the following process would be:

4. the secondary, standby replica becomes the new primary instance

5. the previous primary instance becomes the new secondary, then reboot starts

6. the new secondary(the previous primary instance) is the one that rebooted, so I can assume that the new parameter group should be applied.

7. But the current primary instance, which has just become the primary from the secondary, didn't get a chance to be rebooted, hence doubt if the new parameter group is actually applied on it

But checking the console, the status of the parameter group is shown as in-use. How could this be?