First, for this discussion don't bother about pages being in memory or disk. A page in memory is simply a copy of the page on disk and ASE takes care of the synchronization between memory and disk.
ASE always operates on pages in memory only. If a required page is not in memory, it is read from disk first before anything else can happen If the in-memory page was changed as a result of an operation, it may need to be written back to disk (for a variety of possible reasons). ASE will do that, no need to bother about this.
For how a rollback works, the difference between pages on disk and in memory is irrelevant.
Now, pages that were newly allocated during the transaction to insert many new rows will be freed up again as part of the rollback, that's quite simple.
The time a rollback takes does not primarily depend on whether it is an insert or a delete, but rather on the number of rows modified (inserted/updated/deleted) and the number of pages modified (allocated/deallocated).
HTH,
Rob V.