The term independently corroborated means (to me at least) without control and influence, without outside help, to confirm or give support (verify, endorse, validate). Most likely they have CC footage of BB and RB that confirms dates and times. I am not suggesting they are in no way involved, I just feel too much time is spent trying to twist BB and RB into the SP role. MOO
I don't think RB/BB is the SP. However, if the purpose of those trip(s) were to physically remove themselves from the scene of MB's death, their alibis need to be 100% vetted, not necessarily here on WS but by LE. Are there any behaviors exhibited by either RB/BB that are inconsistent with the purported purpose of their trip(s)? That may indicate a certain knowledge.
ETA - I have no doubt that BB and RB were in Mississippi and California, respectively. I am not convinced that tells the whole story. JMO