Brian Simmons and his Passion [Anti-] Translation: what is ‘the most ancient Aramaic manuscript’ of Galatians?

Galatians 3.15 reads: Ἀδελφοί, κατὰ ἄνθρωπον λέγω· ὅμως ἀνθρώπου κεκυρωμένην διαθήκην οὐδεὶς ἀθετεῖ ἢ ἐπιδιατάσσεται. [NA 28] A typical translation is: ‘To give…