Is db.t3.medium run in unlimited mode?

We're inserting a lot of records into the RDS. During the process, we observed that the CPU utilization is throttled down to 10% after running out the CPU credit balance.

Here's the Cloudwatch metrics:
https://imgur.com/a/P4RGTdM

The insert operations started at around 16:00. The CPU usage was around 100% at the beginning but throttled to 10% at around 18:30. The same moment that we're about to run out of the burst balance.

According to the AWS document https://aws.amazon.com/rds/instance-types :
"Amazon RDS T3 instances are configured for Unlimited mode, which means they can burst beyond the baseline over a 24-hour window for an additional charge."

The RDS t3 instance should be running in "unlimited mode", which should not throttle the CPU usage but charge an additional CPU fee instead when running out the credits?