It's more nuanced than that. Many of those things you mention are backups, just not necessarily that reliable for certain threats. By your standards, you'd probably say tapes, removed from their drive, are a backup. Yet, buildings burn down, taking the tapes with them. If restoring backups from off-line, off-site storage is going to take days or weeks, then for some organisations, you may as well not have a backup. Cloud sync, duplication, connected devices are fine for a backup, but probably not as the only backup. But they still reduce the likelihood of total data loss compared to not having them.
Diversity and risk analysis are what's important here.
I agree with what you say, I'm just nitpicking on differentiating redundancy from backups.
Yes, a building with tapes can burn down. But whether it does so or not is not correlated with the state of the system that the original data lives on.
Redundancy is a great step to prevent data loss. But redundancy won't do what a backup would: keep your data safe even if your system is fucked.
Cloud sync is a double-edged sword, because you make Dropbox a part of your system; their failures are not decoupled from yours anymore. Say, someone hacks into your Dropbox account, encrypts/deletes all data, and then downgrades/cancels your plan so that their backups go poof.
Then you'll find yourself with no data - and no backup.
Compare that to the tape burning down. You just make another tape. The chances of both your system going down, and the tape building having a fire at the same time are astronomically low, because these events are independent.
TL;DR: backups are decoupled, which is why you need them.
I'd say there's an even better distinction: backups keep your data safe even if you manually delete it. With only redundancy if you delete a file it's gone. With a backup, it's still in the last backup. Of course there's usually some maximum retention period for backups, but that can get quite long.
Everything else is secondary and can be covered by both. The chance of your redundant data center burning down at the same time as your primary is also low. If one burns down, you just bring up another. Likewise for smaller-scale users, where the chance of your cloud provider being down when your house burns down is also incredibly low.
So it's really the versioning or snapshot nature that's the key aspect of backups as opposed to simple redundancy.
Agreed that RAID isn't backup, but other online systems, while not completely decoupled, are less-coupled, which serves as a convenient middle ground for some cases.
Diversity and risk analysis are what's important here.