To be clear, they're trying to save more than just storage cost, they're also trying to save time and bandwidth in the backup process. At the moment, both of those things are all local storage to you, so they are fast because you have a lot of bandwidth to your local storage. But if you ever used offsite backup, the deduplication feature would improve the speed of the backup process significantly.

In theory, a single compressed/deduplicated backup like theirs should be just as safe as a single set of file-by-file copies like yours. The only difference is that, to extract a single file from the backup, you need their software to do the extracting.

The redundancy question is a different one. Are you saying that the archive file is a single backup that's been split up amongst multiple storage devices? That doesn't sound right. That sounds like it would increase the risk of data loss rather than decrease it. Are you sure that it's not multiple redundant backups (separate individual complete backups) stored in multiple locations?