I think the argument is more that open sourcing adds incentives and friction that significantly increase the cost of maintaining control/alignment. Or to put it another way, maintaining good design becomes more expensive in an open source model and people respond to economic gradients, changing outcomes at the margin. I've seen this happen consistently, even when limiting "open sourcing" to throwing a tarball over the wall occasionally, it always adds to costs. That burden has to be paid by someone, and not every company or individual can or is willing to absorb it; in many cases the notional benefits of open sourcing are sufficiently limited to be less than the incurred costs.

That said, I am skeptical that many of the points in the post apply to Wolfram specifically, I suspect that is more aspirational on their part. Nonetheless, many of the points are valid as a generalization.