"... 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(Forgive me if my usage of that is no longer current.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."
"... 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(Forgive me if my usage of that is no longer current.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."