I’d be willing to wager that something along these lines happened… Robert and Marie saw newscasts of the Borg scare, mention was made of the Enterprise. After it was all over, and Picard was transitioning back to normal, without his Borg implants, he probably felt compelled to reach out to family, and wrote Marie a letter. Due to their estranged relationship, Robert probably ignored it, and Marie probably read it to him, basically explaining the bare basics of what happened, just so Robert would know how deeply affected his brother was. Just my take.Picard said Robert "knew what happened" and Robert simply said, "not precisely." He knew the basics and he certainly wasn't "too old fashioned" to understand it. But no one except Picard could have known the finer details, which is what he was trying to get at.