One of the hosts of Undisclosed is Rabia Chaudry, who was a childhood friend of Adnan Syed. So really what you are asking here, is whether Ms Chaudry would still have created a podcast without the success of Serial - which is an unanswerable question (but I'd guess yes).
You're quite right about my wording. Agree it's unanswerable but I was also implying the answer would be no. Serial opened a door - for further investigation into the case, and for using a podcast to explore it - that may have remained closed otherwise.