But if not, then what?

On Fresh Air today, Terry Gross said, as she often does, "If you're just joining us, I'm talking with so and so."  But the "if" part is irrelevant.  She's talking with so and so, whether you just joined or not.

A similar phrase I hear a lot is "In case you don't know me, I'm so and so." Same logical problem.

A more accurate statement would be "For the benefit of those just joining us (or those who weren't paying attention), I'm going to say where we got to in today's program before we broke for a message from your local station. We're talking with so and so." 

Or, "For the benefit of those who don't know me, I'll say my name: so and so."

I can see why they use the short, illogical forms.