I am confident that the US has samples of Russian body armor and has performed penetration testing and taken the results into consideration in doing its assessments.
This particular plate may be sub-par relative to how it's supposed to perform. But body armor is not hard to get ahold of; you can't keep it that secret. This isn't going to be a MiG-25/F-15-type scenario, where the US dramatically misassesses capabilities and then goes to build something capable of defeating the (considerably-more-capable) system that it has theorized about. They'll be making a decision based on being able to actually test the system in question.