I think that sometimes calling attention to the subtle "biases" in a person's language is counterproductive as it can make that person feel very self-conscious and resentful that they can't make a simple statement without others reacting to it in a way that had nothing to do with the speaker's intent when making the statement
yes, it may make that person think twice before they say something that you might find offensive - - or it can make them just keep their mouth shut and decide they don't want to talk to you at all. My personal choice ( and yes, this is a choice) would be to say "happy holidays" and take my chances that you aren't overly offended that I didn't say "Merry Christmas" - - although, as it gets to be within a day or two of Christmas, I just might say "merry christmas" - again, with the hope that you won't be overly offended.
by the way, when is Festivus this year?
I say Merry Christmas as that is what I am celebrating. If someone told me Happy Kwanza or Happy Ramaddan (spelling?) I would be just fine with that. It is a sign from that person that they wish good things for me. I would not be offended at all.