My sincere apologies.
Your observation is correct, Selecting a specific IP Address for secured Hub's Firewall is currently not feasible.
You may vote for this feature in Azure Feedback Hub forum .
All the feedback shared in these forums are monitored and reviewed by the Microsoft engineering teams responsible for building Azure
https://feedback.azure.com/d365community/idea/4d8b024a-8926-ec11-b6e6-000d3a4f0789#comments.
Currently, this is the public facing documentation for this : New-AzFirewallHubPublicIpAddress
I shall work internally to get the documentation updated.
Kindly let us know if this helps or you need further assistance on this issue.
Thanks,
Kapil