Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you would like to know if you could use Azure DDOS protection for your custom solution/application in 3rd party cloud.
The FAQ section you have shared basically answers your question.
The 3rd party cloud can be considered as "OnPrem" as long as
- you have taken care of connectivity between the 3rd party cloud and Azure.
- You are able to add the service in the 3rd party cloud behind an Azure Load Balancer (Public IP comes from Azure and users use this for connecting to your service)
If you look at the What is Azure DDoS Protection?
- It is just providing security (DDOS protection) to services that are exposed to Internet via Azure (like Public IP or App Gateway or Load Balancer)
- The backend can be anywhere, as long as it's only accessible via an Azure resource.
- So, by providing DDOS protection to this Azure resource, we are ensuring the protection of the entire application
Hope this addresses your query.
Let me know if you require further information
Cheers,
Kapil