I believe the replies all get the same date as the OP. Not sure on how that interacts with the new ‘preserve chronology’ merge feature, but I’ll give it a test. 
It’s still a bit of a giveaway that it’s not accrued replies in the normal fashion though.
I’ve given it a test run and the replies all got the same timestamp as the OP when it was published. However, when I created a new topic and merged the post into the Scheduled one it didn’t affect the existing order and do anything weird when I selected to preserve the chronology. 