That's not exactly fair. They hold many assets, so of course any action on any individual asset may not have a significant effect. However, if they behave similarly across all similar assets, it will have an effect on the overall fund.
I'd also like someone who understands the issue well to let us know whether these mark downs have any tax implications to begin with. I think it might sometimes be the case that marking down such a holding would allow recognition of a tax loss but I don't think that is going on here.