I think that commenter may have not been that literal and meant something more along the lines of "made podcasts more mainstream in an exponential manner." I know plenty of people in my life who weren't podcast people at that time but were constantly talking about it or walking around saying "mail... keemp?" as a joke. (A reference to Serial)
Serial is the first podcast I can recall being referenced in mainstream network TV pop culture. SNL did skits about it. It was mentioned in dialog on other shpws. BoJack's girlfriend used it as her ringtone.
It was everywhere in a way no other podcast had been previously.
Indeed you’re very right! Not the original, but Serial contained all the elements that made a podcast mainstream and proved it out as a real business. Prior to it they were indie.
Sorry, but this statement is objectively wrong. Podcasts have existed for close to 20 years in the way we understand them today.
Serial may have helped to popularize podcasts, but in no way was original.