ويؤدي المولى المسلم الفطرة عن عبده الكافر
Translation: A Muslim master pays fitrah on behalf of his non-Muslim slave.
The expression names a Muslim owner and a non-Muslim enslaved person in the historical legal framework. The specified liability is attributed to the owner in that relationship.
It does not state that fitrah is a general, direct obligation on every non-Muslim individual.