Hi,
I think I have the same letter that they gave to you.
I've been a full time homemaker since 1997 (I prefer to call myself Director of Domestic Affairs).
I called the IRS helpline and double checked, though I'd taken advice previously, about whether I should be filing anything. I told them that I was repatriating and what was required on the I-864. She asked if I had any income, job or other, and I said no. She then said there was a form letter for people like me stating that if you are below the limit for the relevent year, you are not required to file federal taxes. This woman was very helpful and the letter was posted to me in two days.
I believe the I 864 requires that you write a letter explaining why you were not required to file taxes, if that is the case. So I've done that and will attach the letter from the IRS.
I'm so glad to hear somebody else has the same circumstances as us. I know I'm following the rules, but you can't help but worry anyway. This is reassuring!