You are confusing Inheritance tax (paid by the Estate) and Income tax (paid by you, the beneficiary). Income in respect of a decedent (IRD) is money (such as a retirement plan) was due to the deceased person, and passesl pass through to the recipient. The recipient (beneficiary) must declare the money as income in respect of a decedent (IRD) for any year in which income is received. It that sense, it is different than other inherited assets, such as a house, car, personal property, etc.
https://taxmap.ntis.gov/taxmap/pubs/p559-004.htm
