Yeah it's a stray comma but it doesn't alter the meaning of the sentence at all....

stan_rogers · on Oct 14, 2012

With the comma, the phrase is equivalent to:

"... readers will only read long-form content, and that only if it is excellent."

(Forgive me if my usage of that is no longer current. It has been quite a while since I've spent an extended period of time with current formal writing.) The intended meaning was probably something like this:

"... readers will only put the effort into reading long-form content if it is excellent."

Evbn · on Oct 14, 2012

The grammar Nazi's misuse of "which" in its failed attempt to insult you.