Serial was different, though in that it was a multi episode look at something. This American life had segments and still had to be designed with the radio listener in mind - who was probably only in the car for 30 min. I don't think Serial is that uniquely good - but it was different. We just are kinda dumb for having all of "podcasts" under one term instead of a bunch of genre terms.
As for the earlier part of your comment - it’s there in the name, “serial”. That was the experiment. But there have been serialized radio shows back when radio was the leading medium. We just don’t remember that time.
What I think is key here is that these people had formal experience and mentorship in doing radio. Lots of it. To them it was just applying that expertise to something we call something different due to shifts in distribution tech.
Apple didn't coin the term. People started using the word in 2004, but iTunes didn't have podcast support until 2005.