Using related pair of cryptographic keys to open/seal plaintext
You are effectively using symmetric encryption.

The crypto_box function uses elliptic curve Diffie–Hellman on Curve25519. With a given input private key and public key it always generates the same symmetric key, which is then used for authenticated encryption. By using the private and public key from the same key pair you are generating the point $a^2G$, which with a random $a$ is just a random point on the curve. This point is then used to derive a symmetric key for authenticated encryption.

Of course, it would be more efficient to just use that derived key (or a random key) in the first place, but as long as you keep the key pair secret, what you are doing is secure.