I disagree with this statement from Samsung, even if it was a safe option then it is still not the safest option. Offline storage is considered safe, but that is not really the case with phones. The purpose of a portable phone is to communicate with others over distances, so you are always connected to some sort of telephone network. New smart phones also include different wireless communication functionalities like WiFi, which stops you from being offline. Malware could potentially access your phone via WiFi or other forms of communication and steal your private keys. The most safe option for storage is really a paper wallet, digital devices with wireless communication functionalities cannot compete with that.