If an incremental Snapsync happens with "RMAN backup Plus archive logs", why do you need archive logs generated for the period when the backup happens?
Delphix takes full backups, and differential incrementals. All datafile changes before the given checkpoint SCN (system commit number) are guaranteed to be written to a given datafile. A backup of a datafile also has a "fuzzy" SCN, which is the highest SCN of the blocks contained within the backup received. This is stored in the datafile header.
Recovery of each datafile in the backup needs to be able to reach the highest SCN in order for the datafile to be usable. Without this, the backup is ultimately inconsistent, as the datafiles are changing during the backup. Datafiles change during the course of an online backup (one of the necessary consequences of taking online backups vs. more disruptive methods which freeze all changes during the backup window).
In order for the backup to be usable, Delphix collects archive and redo logs from the source to cover the range of time from beginning to end of backup (SnapSync).
There are two ways to handle this backup concern; the backup can be initiated with a single command to include backup of data files and archive logs, or they can be issued in two distinct operations with range specified; Delphix uses the latter method, which provides the benefit of defining the range of archive logs needed applicable to the SCN range which the backup window covers. The former method would fetch ALL archivelogs, and consumes additional storage and redundant collection (assumes RMAN backup optimization is not enabled)
Ultimately, Delphix requires archive logs covering the range of time for the datafile backup be available and stored in Delphix for a given snapshot to be consistent and provisionable. This is the choice that has been made for consistent backup and recoverable snapshots with efficient backup operation and storage considerations.
Oracle also provides details on backup and recovery methodologies, and this can be reviewed with your team as desired. For more information, you can also review the RMAN operations executed.