Subject: Re: Numbers
There is a cap of 60% of AGI (and in some circumstances lower) on cash contributions to public charities.

You are correct. IRS allowed 100% deduction only for tax years 2020 & 2021 to encourage donations during the pandemic and it has since expired. Anyhow Warren is probably not concerned about tax deductibility at this stage (he is not even using the write off from his stock donations) but probably just wants to make sure the charities get 100% of monetary value of his donations.