First, and primarily, Tim Sweeny states: "If this tax scheme had been place, I’d have been forced to liquidate nearly my entire ownership." This is absolutely false.
The proposed law (legislative text available here: https://www.finance.senate.gov/chairmans-news/wyden-unveils-...) would not apply the tax scheme to Epic Games. At all.
See Section 491, which applies the new tax scheme to "tradable covered assets", and Section 497 which defines "Tradable Covered Assets". Those are defined assets traded on established securities markets or readily available on secondary markets. Epic Games is a privately held company, and not traded on established securities markets or readily available on secondary markets.
Additionally, the law allows each person to designate up to $1B of normally "tradable covered assets" as "nontradable covered assets". Which means if this law did apply to Tim Sweeny, it wouldn't have impacted his holdings by as much as he implies, as $1B of his assets is removed from his "assets" at the start of the calculation. This significantly reduces the amount of shares he would have to liquidate to cover his take burden.