How to Improve Page Authority of Shortened URL Backlinks?

Before 2 days I was analysing a page in MOZ OSE and I surprised that a page has many 301 redirected URL shortened backlinks with almost PA-46 then I analysed many other pages of the domain and some another big SEO experts page and they are also having the same myth. Now I am very confused that how they have that metric without any link pointing to those URL shortened URLs I am attaching a pic also. Kindly clear my confusion.

Interesting topic I am searching around but I think without test we can not talk about results.