That article you posted does explain it at the end. It says....
That resolves it. The above-the-line deduction for 2020 is capped at $300 per tax return whether you’re single or married filing jointly. It’s capped at $150 each if you’re married filing separately.
[Update] Just to make it more confusing, Congress passed a new law that extended this deduction to 2021, and the cap in 2021 for married filing jointly is increased to $600. The cap in 2020 for married filing jointly is still left at $300. See 2021 $300 Charity Deduction For Non-Itemizers $600 Married.
