I believe they determined their methodology on past similar studies (at least they compared their results, so I would hope they used the same methodology for determining a degree). The degree of separation just asserts that there exists a friend 1 who is somehow connected to another person (and on average some distance from any other person). It makes no claim on the quality of those connections.

Also, the assumption that because two people have interacted means those relationship are easier to leverage isn't substantiated. I'm sure the result would be interesting, but it wouldn't necessarily be "more correct".