It's possible to sell those shares on a secondary market, i.e. seed investors might have already cashed out by now.
http://fortune.com/2014/06/20/uber-plays-hardball-with-early...
I think the lockdown happened after that.
http://techcrunch.com/2011/02/27/jp-morgan-twitter-chris-sac...
He may have done the same for Uber, but I haven't heard anything about that at all.
"the Uber CEO got upset with Sacca for trying to repeat his Twitter move of buying up secondary shares in Uber from other initial investors."