My reading of the quoted language—
Having reviewed the posts that you have identified, I conclude that you do not have any non-frivolous defamation against any of the posters.
is that it is intended as a short form of this language:
Having reviewed the posts that you have identified, I conclude that you do not have legal grounds to pursue any non-frivolous action for defamation against any of the posters.
In other words, you can pursue an action for defamation in court, but any sensible judge will conclude that your action is frivolous—that is, according to definition 1b of frivolous in Merriam-Webster's Eleventh Collegiate Dictionary (2003), "having no sound basis (as in fact or law)."
So the lawyer (or pseudo-lawyer) is telling the client (or pseudo-client) that any proposed action for defamation based on the specified online posts is without merit and will fail in court.