"is it possible that the estate had sufficient deductible expenses that the taxable income for box 5 was reduced to zero?"
It's possible. It depends on the amounts. Of course if the person preparing the estate's Form 1041 simply failed to enter the taxable amount of the distribution, that means that the estate filed in incorrect tax return and would need to amend.
There is one other possibility: If the estate uses a fiscal tax year rather than a calendar tax year and the distribution from the inherited IRA occurred during the estate's 2025 tax year (which would begin in 2024 and end in 2025), the income should be present on the next Schedule K-1 that you will receive which you would report on your 2025 tax return.
Either way, since you were told to just add it as other income on your tax return, I suspect that the estate's income tax return was simply prepared incorrectly and there should have been a taxable amount in box 5 of your Schedule K-1. That suggestion definitely seems to indicate some lack of understanding on the part of the executor as to how the income should be (or was) reported on the estate's income tax return.
