Whether you believe his reasoning or not, I think it's an entirely valid reason to sell. He needed cash, his entire liquidity is in $TWTR. What else is he going to do if he needs personal cash, take a loan?
You could still run into issues. Imagine twitter stock crashed and you took out a large loan, you're still on the hook for that amount and now you don't have any assets. I would also assume that the ventures that he claims to be going into (charitable donations/VC funding) would require you to have the assets in hand.
I believe the thing about innovation in the field of medicine is that it has a lot at stake when trying new things. From theory to wide practice there are a lot of barriers, impeding the development of the things you mention.
But there is something certain, technology will play a enormous role in medicine. You shouldn't be worried about whether or not we will see such things in a 10 year lapse.
