That's not true. There are definitely security benefits of separating funds. Even with hardware wallets which I pointed out above. Although, some are a little more hardened than others. The point is the vast majority probably don't have enough to justify going through these additional steps to secure their Bitcoin.

If you've got a completely offline wallet, then you could argue there's no risk of losing it. However, that entirely depends on how you're storing the back up. Separating these doesn't necessarily mean you'll keep the second copy any more secure than the other.

However, there are definitely certain circumstances which separating funds will prove to be beneficial from a security stand point.