Sure, but how does that apply in this case? Roth account legislation was passed in 1997, and PayPal was started in Dec 1998.
There are so many ways to avoid paying taxes if you are wealthy, only a few of which have been publicized (Warren Buffett vs secretary, Mitt Romney and the Bain Capital leak, Panama Papers revelations, etc.)
Regular people don't have those options; and a vastly greater percentage of their income (which is directly taxed already) goes into local taxes on gas, essentials, and any luxury purchases.
When we see another reminder of how unfair these systems are, and we know that their design comes from the people who benefit from them (like Thiel), it is reasonable to direct our ire toward them.
And finally, this doesn't even touch on the overt criminal tax avoidance that is well known to persist amongst this group. The Swiss bank leaks exposed some of these, and the US tax amnesty programs have further illustrated it. Even so, there's surely a lot more money hiding away that has not and never will be taxed.