The bottom line is that the practical definition of a cappella changes. Whether or not "purists" like it, and whether or not the dictionary reflects it, it now includes contemporary a cappella with all of its (live and studio) processing, pitch shifting, octavizing, screaming, and percussion. Why? Because, for years now, people have been referring to that stuff as "a cappella." If I start thousands of sackbut ensembles across the world and successfully get people and media to refer to it as "a cappella" for years on end, then, by god, THAT'S a cappella.
And saying that the typical Western mid-20th-century a cappella aesthetic is real/true/definitive a cappella is like being from an American family that immigrated from Europe 100 years ago and now saying that immigrants are ruining America. If you're going to be "pure" be really pure and go sing Stabat Mater in the Sistine Chapel.