Yes, the legs on the Osmo Base for Fire tablet do need to be extended when using the Amazon Fire HD 8, 10th Generation (released in 2020). In order to extend the base legs, you need to pull the legs out while keeping the buttons pressed.
Please refer to the following video to see a demonstration of extending the base legs:
https://youtu.be/DsW_RiIAoDc
The instructions for whether to extend or shorten the base legs are also part of the setup steps presented at the launch of each app.