if by 'lie' you mean part of the culturally specific social ritual which is an essential glue knitting disparate people together, then yes, it's a lie.
we lie all the time to knit people together, yes, of course. The key is in the "culturally specific" part. What kinds of lies are coded as rude and what kinds of lies are coded as glue? This differs, which was all I'm saying. I don't want to take the ritual from the English; I just want to keep them from making it universal (the English are very good at making whatever they do universal around the globe. ahem)
This isn't just different between cultures, it's different between status positions, scenes, relationship levels. If someone I'm just getting to know asks me for lunch and I don't feel like it, I'll say "I'd love to, I can't" if I do want to keep making a relationship with that person. If someone's an old friend, I'll say, "I don't want to", knowing that our friendship won't be hurt by that.
This isn't just different between cultures, it's different between status positions, scenes, relationship levels. If someone I'm just getting to know asks me for lunch and I don't feel like it, I'll say "I'd love to, I can't" if I do want to keep making a relationship with that person. If someone's an old friend, I'll say, "I don't want to", knowing that our friendship won't be hurt by that.