I ran the code and got the same results. The link is not relative. If you open the file in Excel and right-click on that cell and pick edit, you will see that the path is a full path. I was not saying that I can't supply a relative path for the Address property of the Hyperlink, what I was saying was that your class code changes it to a non-relative path.
If you create a Hyperlink through Excel itself, I can specify a relative path. Attached is an example showing what I get with your code (cell B2), and you will see that in cell A1 I have created a truly relative hyperlink.
after.zip