I agree with you, I was way out of line when I called him a lier. The fanboy-ism may be a little to strong in me.
But, to the point, he never said how much traffic he was seeing, so in his case /any/ traffic could be deemed "a lot". All services these days use your network connection to some extent, but I'm finding it hard to believe that it actually may end up congesting your networking connection, without seeing some serious proof.
I can to some extent see his point where this should've been clear from the start of, that Spotify will make use of your upstream network connection. Hopefully Spotify will make changes so that at least the US-market get notified that they might end up paying their service provider a premium, just for using Spotify.