Note that this was an unusual arrangement, to say the least. My understanding, from memory of having briefly looked into this, is that Acton loaned $50m to the Foundation, rather than donating it, in what appears to be a 50-year, interest-free loan with no regular repayments. As an initial donation of that size from an individual would have probably put the Signal Foundation into private foundation status rather than public charity status, this has at least the appearance of trying to circumvent the public support requirements of public charities.
It is somewhat difficult for one individual to consistently single-handedly support a charity in the US without causing the tax status of the organization to change detrimentally.
There are provisions for individual large donations to be excluded from the public-support calculation if the charity can make the case that they are "unusual". No idea if that would apply here though.
But that---a $50m donation from an individual, then sustaining the organization off investing that donation---is exactly what, whether it makes sense or not, a public charity in the US is usually not allowed to do. That would make it a private foundation in the eyes of the IRS.
With that said, if I'm interpreting their 2019 filing correctly, it appears that they are making enough in donations that they may have a legitimate claim to being able to eventually repay the loan, and they are now including imputed interest on the (interest-free) loan as revenue.
It is somewhat difficult for one individual to consistently single-handedly support a charity in the US without causing the tax status of the organization to change detrimentally.