Don't know how that is possible as it is programmed to treat shoulder exactly like peak
That table is totally wrong for Cost Savings mode. This is not an official Tesla chart.
It's more complex than that. From the actual Tesla manual:
Powerwall is configured to meet your home’s specific energy needs. Use the Tesla app to monitor and manage your solar system’s performance with features and control modes such as Backup Reserve, Self-Powered and Time-Based Control.
www.tesla.com
Basically it will decide when and how to discharge in Cost Savings mode with the intent to get you the best possible price for your power vs how much you have to spend. In may setup and usage, that almost always means it does not discharge during shoulder. The exception is when it thinks that it will fill the batteries sooner during off peak on the next off peak period and therefore selling back during partial would be better than giving it away during peak.
Each users confirguration and usage will change the behavior. This is why they put this disclaimer on it: