Interesting research and the conclusion certainly makes sense.

I would add:

When 5 names comprise over 20% of one's benchmark, it is hard to overweight those 5 names even if the portfolio manager believes that they will outperform.

Active portfolio managers tend to construct portfolios that roughly "mirror" the underlying index and then make relatively small overweight/underweight bets that do not "move the needle".

Active portfolio managers often have diversified portfolios with too many names to allow for meaningful outperformance, if there is any outperformance at all. A former colleague used to refer to the practice as "diworsification". (mispelling is intentional)

I keep hearing active managers proclaim that the time for active management outperformance has arrived.

It appears as if that train is late again.