But if your system is to pick high variance players that are also underrepresented on other players rosters how many lineups can there be? I remain skeptical that this is a viable strategy.
You _must_ pick some people that are high-cost high-reward types. The superstars that will almost always get their lion's share of points. You cannot win a tournament just by picking high variance players, or it would be extremely hard to do so. You also have a salary cap so you can't just pick 5 superstar players. The higher their perceived worth, the higher they cost to "buy" them.
So the main point you can differentiate yourself from other players is to find "sleeper" picks. These are players that are OK but not GREAT but if given a proper matchup, could do very well.
For example, you may pick the 2nd best receiver on a team hoping he has a great day because the opposing defense is known to double team the offense's best receiver. This may give the 2nd best receiver more opportunities for catching the ball for yardage/touchdowns.
BUT the thing is, this happens a lot. So you know maybe 4 games where this will happen. But you don't know the percentage picked of each player. Just like in March Madness brackets where picking all chalk (the favorites) to win is a bad strategy because almost everyone else is doing it and at best/worst you will tie, it is to your advantage to bet on players that you think _may_ have a chance, but are picked the least from the pool.
So the advantage this employee got, was he knew how often each player was picked on DraftKings, so given his basic assumptions he just applied those percentages to FanDuel. He knew player X had a good matchup, but _no_ one was picking him. So he plays multiple lineups with player X, buying him up so if he has a good day the employee will win big.
But he also places wagers on player Y, player Y has been picked more than player X, but still enough where the employee can get a good return, so he places a couple of lineups with player Y in it (even combining with player X hoping they both have big games). Buying some lineups with player Y also hedges your bet in case player X doesn't do well. This is probably what you are curious about. You don't pick only player X hoping he does have a great game. You hedge by buying more lineups with him in it, but supplement that with other lineups and picks that you think are +EV.
So the advantage is you know who hasn't been picked a lot. And as you start buying players you have to start spending more "efficiently." If you know player Z has only been bought 10% of the time but you know he has a high likelihood of a big game, you will pick player Z more in your lineups than other lower priced players because his returns will be good.