Why does SQL Server require non-clustered index to use unique keys? According to https://stackoverflow.com/questions/4332982/do-clustered-indexes-have-to-be-unique

The answer is yes. In the sense that SQL Server will make it unique by appending 4-bytes to key. This makes sense given that clustered index is used for data addressing purposes (Correct?).

According to my research, it seems that SQL Server requires non-clustered indexes to be unique as well. And when non-unique column is used, SQL Server appends a 4-byte "uniquefier" value to make it unique. Is that correct?

Why does SQL Server require non-clustered index to use unique keys?
Probably it simplifies the engine a lot because you can now address individual rows.

Imagine, you are indexing on a IsMale bit column and there are 1 million "male" values. Now you want to address exactly one of those rows to delete it. The index has 1 million rows that just say true and nothing more. I'm sure it's possible but uniqueness makes the design much more elegant.

Locking requires index rows to be uniquely identifiable.