I think Lister was drinking more than usual. BtE strongly suggested Kryten told Lister she was flushed out the airlock. I guess Kochanski could have commanded Kryten to lie , but that wasn't really my impression. I think Kryten wanted to spare Lister the knowledge she had left him again... although I'm unclear why news of her death would be better... Was Kryten still jealous and saw this as a selfish opportunity to keep them apart? Lister would likely have chased her if he'd known she had left. I sort of got the impression that the Kryten of BtE had got past that silliness of Series VII though. There is certainly plenty of stuff left untold.