This is not true, it has no effect on the current valuation at all. Softbank are looking to buy shares at this lower valuation since they are a different class of share to what they would get otherwise; these are common stock (employees will be a major source of stock for the sale) and therefore don't have the same benefits of preferred stock, of course they expect a better price. Where they are receiving preferred shares (the $1B investment) they are using the $69B valuation from Uber's last round (presumably this $1B is being added as a late-comer to that round).
I can't believe that a Bloomberg reporter would get this wrong, so this sounds a lot like jumping on the media band wagon. Suggesting Uber has lost 30% of its value is blatantly false.
Edit: It's fair to say there might be some impact on the notional valuation between rounds, which is hazy at the best of times. However it's not a 30% cut.