How can I get Siri to set my Nanoleaf light back to adaptive lighting?
Siri doesn't know how to do this (as of iOS 14.4, because the site you linked to is for iOS).  You need to first create a Scene that sets the light to adaptive lighting.  Then you can tell Siri to execute that Scene.
These directions work on any device that has the Home app.

From the Home app, tap the + icon (top right) and select Add Scene.
Select Custom.
Name the scene to use the phrase you want to use to turn on adaptive lighting.  I used &quot;Turn on adaptive lighting&quot;.
Tap Add Accessories.
Select the light(s) to control (in your case, it will be your Nanoleaf bulb).  Tap Done when finished.
Under Accessories you'll see the Nanoleaf bulb.  Long-press the device to show the color configuration screen.  Tap the adaptive lighting icon which is the first icon on the top row, left.  Set the brightness as desired.
Tap the X to close the screen.

Now, you just have to give Siri the name of the scene and it will work.