exchange12rocks a last note just to avoid misunderstandings by other readers.
Could you please point me out where in the official documentation it says that Az Stack HCI can be free?
That you have doubts it shows that the communication about it could need improvements. I have previously linked a webpage explaining licensing and more.
To answer your question:
1. You can find it in the docs explaining Windows Server Azure Hybrid Benefits and Azure Stack HCI Hybrid Benefits. As well as on Product Terms.
Unfortunately the list of AHB is not yet exhaustive, currently not naming Azure Update Manager while it is included but awkwardly only mentioned in Azure Update Manager docs.
2. Please consider to have a second look on the same pricing page you have linked.
"Price with Azure Hybrid Benefit"
Azure Stack HCI and also Windows Server VMs are for 0$ (as they usually are with Datacenter) when using Azure Hybrid Benefits rights from your existing licensing with Datacenter + SA or CSP Subscription.
If you like to learn more, drop me a DM.