If you havenﾂ’t done already, that is.

I've just came across an expression, "If you haven't done already, that Is.".

There seems to be a similar usage such as in the following site:
Marcos Maidana vs. Lucas Matthysse: A Storm is Brewing

"It appears that Schaefer and Showtime will soon be having very similar discussions, if they haven't already that is." (end of the paragraph.)

What does this "that is." mean?

It's modifying the previous clause.

You could change the last clause to 'that is if they...' and it would have the same meaning.

Essentially, the pattern is: A is true, if B isn't that is.

Oh, I see it now. Thank you for the clarification.