If Fidelity just did a 25% write down on SnapChat on the most senior portion of a $600m investment round, and assuming that Fidelity has at least a 1x liquidity pref/ratchet, then SnapChat is now valued at $462M floor, not $15 billion.
If they are guaranteed to at least get their money back AND they did a 25% write down, then they are assuming that if there was a liquidity event tomorrow, that that would only get back $0.75 on the dollar, and others less senior get $0. This means that SnapChats valuation mark-to-market price is only $462M because that is all they feel that could be recouped.