Kinda, sorta.
Agreed that fund managers must operate within the defining parameters of the fund they manage. And, agreed that funds are limited by the SEC and the fund's definition regarding holdings.
However, whether active fund management is a turd or diamond depends on which way you are defining turds and diamonds.
The Wilshire 5000 is the broadest index of equities in the U.S. Investors get the "most average" returns. Fund managers assert that through their research, analysis, tossing of chicken bones, etc. that they can put together a subset of stocks that is smaller than the Wilshire 5000 but with a higher rate of return at lower risk.
An example is the Janus 20 fund that is limited to 20 to 30 stocks. If they only have a couple dozen different stocks, then the managers better pick very carefully. For a time, Janus 20 was going pretty good.
Managers have a higher likelihood of finding the diamond in the rough in the smaller cap stocks. For example, a management team may look at several different gold mines. They will look at labor relations, the quality and rights to ore, production costs, etc. For small companies, that one team may be the only interest in the company. On both a qualitative and a quantitative basis, the team should be able to select one gold mine over the other. Nonetheless, the team has still picked a gold mine and remains subject to the larger market/economy. Hopefully, they may have either picked the biggest winner or the least loser.
Bill Miller at Legg Mason, Peter Lynch at Magellan Fund, and Warren Buffet are just a few examples where a managed fund has not only beaten the corresponding indexes (at least for some period of time), but also lead their respective investment categories. So, they could be legitimate diamonds.
The successful managers also become a victims of their own success. Money pours in as new investors try for a piece of the action, and there is no desirable place to invest it. The fund managers have to put it somewhere. That is why you see new, small funds on the cover of Money magazine post above market rates of return but over time the rates of return return to the mean.
Turning to the turds, 75% or so of the managed funds lose to their corresponding index fund. To be blunt, those managers have shown themselves to be turds during a time when their clientele could have done better with an index.
Everything is subject to the whims of the economy. One cannot get outside the economy.