You are mixing up apples and oranges .... there is an estate tax return which has an exemption up to $12.06m (form 706 https://www.irs.gov/forms-pubs/about-form-706 ) and the estate INCOME tax return (form 1041) which has an exemption of only $600. So if the estate has income in excess of $600 then an income tax return must be filed.
