Marshall was an amazing bowler, and would be in my all-time XI (and Hadlee, too).
But I can see Benaud's logic here.
He takes a champion spinner, which is understandable (for one moment, not going into the debate about which champion spinner he takes and why). Sobers complements that champion spinner as a part-timer with completely different variety.
Benaud takes Imran who is among the best of bowlers, and adds immense value in terms of battting depth.
He takes Barnes either because he thinks he's the best of bowlers, or because he thinks he adds exceptional variety to the attack. Both of these reasons are understandable.
Then he takes Lillee as a fast bowler. Now, I or you might rate Marshall a better bowler than Lillee, but there are many who played with or against at least one of them rate Lillee as the better bowler. Lillee was extremely successful in the World Series cricket, which is considered by many the highest level of cricket ever played. Lillee's test bowling stats are exceptional, but a shade below Marshall's. But there's a high chance that he was an even better bowler than what those stats suggest. Sobers rates Lillee as the best fast bowler ever, Hadlee considered Lillee his idol. And there are many many other champion players who rate Lillee as the best fast bowler ever.