Pardon my ignorance, but hasn't it been trivially known why orninthine and DAB don't make proteins? This was a test question in undergrad bioorganic chemistrty class 15 years ago. Ornithine can self-cyclize to make a six membered lactam, and DAB will self-cyclize to make a five membered lactam, which are energetically favorable, and thus chain termination compete with chain extension polymerization, making them unsuitable for proteinogenesis.

edit: read the paper carefully. They came to the same conclusion. But... Is this new?