I don't understand the need to make up something that still reinforces that the elf is real..
"Oh he doesn't come to our house" - surely that would make children more unhappy?
For me, its a simple chat, no need to treat it as if it is real, but also no need to outright say it isn't. Just an explanation that every family is different, and we all have different traditions, and aren't we all so lucky to have lovely Christmas traditions that we enjoy.
But then, my son is growing up outside of the U.K. in a community with a lot of different families and cultures, so its a bit more extreme here. Still though, each child needs to learn that their are some things their family does, and some that they don't, and that is the same with all families.
It has just been thanks giving, and the American families we know have been celebrating, but we didn't.. When it is Christmas, some of the families will not celebrate for religious reasons, and those that do will have all different traditions because of the range of cultures. Among the English families there are still a lot of differences that the kids notice/talk about, stockings, presents, what comes from santa, advent calendars, etc.
It is all fine - no excuses needed. Families are unique - children can understand that.