I've always thought that was a misnomer; there isn't much about the Sunday talks that is truly aimed at the public.
When the talk schedule is being arranged, elders are directed to review the titles and schedule talks that meet the needs of the local congregation. When phoning other congos to secure visiting speakers to fill out the talk schedule, it is not uncommon for the guy on the other end of the phone line to ask, "Are there any local needs or issues you want me to touch on in the talk?"
And the talks are always prepared with the congo in mind. Local "problems" are often addressed in a general way during the discourse, and sometimes it's not even that subtle. The talk is often viewed by the elder doing the talking as a opportunity to straighten out certain matters. Finally, "public" talks are laced with "in-house" language that only dubs can relate to.
So, no, there's no such thing as a public talk. And dubs don't generally invite members of the public to hear these talks like they used to 30 years ago.