OTP is safer than the open asymmetric open key approach. Open keys are used in almost all other messengers. But the open key secret is theoretically solvable, it means it can be broken by a third party if it has enough time and computing power. Anyway, it may be compromised very fast in post-quantum reality. A part in the middle can keep your message for years and then open it in seconds if quantum computers become available.
Exchange the keys
If the OTP is so safe, why everybody around does not use it? The problem with the OTP is the key exchange procedures. You must share your key with someone, during a personal face to face meeting, and never use the internet or local network to share it. That’s why the Cryptogram app creates a peer-to-peer protected TLS connection with an effective range up to 5-8 meters between the two nearby devices and exchanges the keys over this connection. Cryptogram uses neither your local Wi-Fi network nor mobile connection to send the keys, but the Wi-Fi module of your iPhone must be active to provide the tool for the secure peer-to-peer connection between the two devices. The password is available over the onscreen QR-code. Nobody else within or beyond those 8 meters can obtain the password and receive the keys, but only the partner who has scanned your QR-code at the moment.
Length of message
The message length is limited. To provide perfect secrecy, the key must be longer than the text. The envelop you send is always wider than your message. It also contains some nonce and consistency code. Nobody can know the real length of you message inside the envelope or spoil it. Thus the length of your text can not be more than 200 symbols.