Granted, the setting is the Patriarchal period and this was before the Torah of Moses was revealed. That could be why ritual law is ignored. But the Book of Jubilees covers the Patriarchal period and is still full of interest in the ritual law. And the Testament of Zebulon even anachronistically mentions the Law of Moses (3:4). So I am not entirely satisfied with that explanation.
It is clear that some of the Testaments drew on Jewish sources in Hebrew and Aramaic, but I don't know whether all of them are based on such sources. If so, a lot of those sources are lost. Some may be Christian compositions written to fill in the gaps to make of full set of twelve testaments.
I have noted earlier posts in Phil Long's blog series on the Old Testament Pseudepigrapha here and links. His current series is on the Greek Testaments of the Twelve Patriarchs. Joseph is number eleven. One more to go. Cross-file under Old Testament Pseudepigrapha Watch.
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