Here is the misunderstanding: "According to the IRS, they base taxpayer eligibility for EIP checks on 2018 or 2019 returns per IRS guidance"

The Checks are an advance against the 2020 tax filing credit. The 2018 taxes are moot and do not apply. The 2019 taxes are moot and do not apply. No one has been injured or otherwise affected by the Corona Virus or ill with Covid-19 in 2018 or 2019. All of this was how they tried to figure who gets mailed or direct deposited how much money.

Only 2020 is impacted by the virus.

And any tax year filing should be accurate.