To put it more clearly.

Your piece of paper or steel with a secret/private key written on it is your property.

One does not and cannot claim property on the specific arrangement of words in someone else’s property (paper/steel).

If someone simply sees your seed phrase and writes it down without committing any crime such as breaking into your home, then they did not steal your property. After all you would still have your piece of paper/steel which is what you own.