When you obtain an EIN for the estate (necessary to open the estate account), you tell the IRS the whether the estate will be using a calendar tax year or a fiscal tax year. You apparently indicated that the estate would be using a calendar year, so the IRS simply sent you a reminder that the filing deadline for a calendar-year estate is April 17,2018. It's doesn't in any way indicate that the estate has received enough income that it is required to file an income tax return.
Even if the estate has received sufficient income to be required to file, the selection of calendar or fiscal year when obtaining the EIN does not actually establish the income tax year of the estate. That's done when the estate's first income tax return is filed. (§1.441-1(c)(1)) When obtaining the EIN for the estate, it seems that the IRS only asks for the anticipated estate income tax year to allow the IRS to send the filing-deadline reminder at the appropriate time.
