I think the percent of holdings argument is going to apply to these funds at Fidelity even if you take all the startups they own together. I guess "tens of millions" becomes excessively dismissive though.
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.