he added of some of his big-name opposition.

[Maybo]

"People had kind of heard about it before but no one really knew about it. And then as soon as you said to someone who knew football, they were like: 'Oh wow, you're going to the Super League.'
"I have no regrets about it at all. It was amazing."
"You kind of have to just completely forget who they are," he added of some of his big-name opposition.

Why do we need "of"?

Source: Chinese Super League: From bidding for Bale to selling the team bus by Sophie Williams

 

[5jj]

He did not add some of his big-name opposition. He added a comment about them.

 

[emsr2d2]

You can take "he added of some of his big-name competition" to mean "he added, talking of some of his ...". As 5jj said, "of" here means "about".