Yes, you're right. Radios were technically not standard equipment on Mustangs, at least for the first few years. You always see a line item for a radio on the window sticker with associated cost. Delete items, where a credit was given if not installed, were limited to things like heaters and seat belts.
For something like radios, though, it wouldn't be uncommon for dealers to order them without radios and install aftermarket kits that looked like the factory radios. So, even though the car didn't come with it from the assembly plant, it could have been sold with one when new.