Is iCloud a backup or a coordination mechanism? If someone steals an iDevice and starts nuking notes or deleting videos, are they not gone?

It's a tradeoff of convenience vs. security. If someone steals my iPhone they would first have to get into it. Assuming they can do that, I'd have much bigger problems than my notes being tampered with.

Additionally, iOS devices can be locked remotely.

All deleted notes go to a "recently deleted" folder in which they will stay for 30 days. Although they could be permanently deleted from there and then it is really gone...