My assumption is that they owed no tax. IF they owed no tax it would have been refunded.
Had they owed, the IRS would have retained any amounts necessary to pay any tax due. Of course, that is also not a bad thing and means that you are not subject to interest and penalty, to liens and seizures, lawyers bills for lawsuits, public humiliation when your name is in the local newspaper as a tax scofflaw, comparisons to Cheat’n Charlie Rangle, etc.
It is also imperative that once you do this you ALWAYS file within 3 years of the due date for that tax year, and confirm that your return was received. The Refund Expiration Date is three years after the due date and they will NOT refund any prepaid credits to you if you file your return after that date!
We filed for an extension after about 3 months after her death, and sent with that a check for $245K.
A year later we filed the final form 706 which called for a refund of $68K, which they promptly sent us.
Several months later we were informed that the estate filing had been chosen for an audit. That process resulted in a year-long ordeal of getting things re-appraised, getting old bank statements and so on.
The audit resulted in a reduction of the overall estate value. They did nail it for "gift taxes from the 1970's" but otherwise was quite favorable.
The final refund check did actually include about $4K in interest that the IRS gave back to us... which surprised me.