It is an interesting theory. The classic antitrust concern is that companies can use their monopoly or near-monopoly position to unfairly benefit themselves (to the detriment of consumers). If a single company or a collection of companies acting in concert is able to corner a market, then, in the absence of antitrust regulation, that company or collection of companies can price gouge, which benefits that company or each of the companies in the collusion (but obviously hurts the consumers in that market).
Here, though, the argument seems to be that companies with sufficiently high overlap in their institutional ownership may each seek to maximize the profits of the entire industry rather than its own profits, even at the cost of sacrificing its own profits. It is true that such behavior would be in the interest of shareholders whose ownership spans the entire industry, but, in my view, it is absurd to believe that managers would (or do) consciously try to maximize total industry profits by sacrificing the profits of their own company (unlike traditional collusive behavior, where each company participating in the collusion is seeking to maximize its own profits by maximizing the profits of the entire industry or collusion of companies).
Imagine a company's management trying to justify such behavior to its shareholders. "Yes, we, Coca-Cola, could have competed harder and increased the price of your KO shares, but we didn't because we know you own Pepsi too!"
The underlying premise is that companies may act in the interest of its shareholders, not considered in their capacity as its own shareholders but in light of their broader diversified shareholdings (which is why, the argument goes, companies may seek to maximize total industry value rather than individual company value). You can extend the premise of this argument to further logical extremes: after all, companies know that some of its shareholders are customers of the industry too, so why not take those interests into account? If you zoom out far enough, you will find a common denominator across every shareholder of every company in the world.