That keeps getting brought up and makes zero sense from a software development perspective.
genie wasn’t in place before either. So any second system brought up could have been their old code base updated too
I mean, it's been stated by insiders that this was the case, so...
And you're partially right - it doesn't make sense from a Software development perspective if said software system is developed well. But let me present an example:
Warning: tech talk ahead
First, let's say they repurposed the APIs. Now MDE is using the APIs for FP+ for park reservations - and they repurposed classes within MDE that were used for FP for FP.
Now - let's suppose that the backend code had hardcoded references to databases, other systems, etc littered throughout the code. Bad practice, yes, but I've seen it done. So now all the internal systems, databases, APIs, etc from FP are now used for Park reservations.
So now they are bringing back some form of line skip, - at minimum they need new APIs, new classes in MDE, and new database objects to hold all of the data. If their system is poorly architected and contains a lot of leaky abstractions, it could be a huge undertaking to reimplement the system.
It's also worth remembering that up until recently, Genie had more elements of FP+ in it (for example, the ability to schedule times). Apparently that was removed somewhat late, my guess is because they were behind, had a business goal to get Genie+ out by EOY on both coasts, and it made sense to share code already developed for DLR.