Romans 3:
[1] Then what advantage has the Jew? Or what is the value of circumcision?
[2] Much in every way. To begin with, the Jews are entrusted with the oracles of God.
To add to the oracles of God given to the Jews is to defy the will of God.
The Septuagint, including Tobit, was compiled by Jews from among their holiest writings. It was the closest thing the Jews had to canon, as demonstrated by the strong preference for it in New Testament quotations, until the anti-Christian Pharisees decided, sometime around the turn of the first century, that they needed to reboot the canon by excluding anything which did not meet their carefully contrived arbitrary exclusions.
Preferring the canon of the Pharisees to the canon of the New Testament writers is indeed a strange alley down which to be led in defence of Chritian scriptural authority.