Yeah but the claim is "Alas, its arithmetic coder isn't bijective-- so you can't just decompress encrypted input back into english." which is an odd thing to sa

Yeah but the claim is "Alas, its arithmetic coder isn't bijective-- so you can't just decompress encrypted input back into english." which is an odd thing to say: this can decompress anything that it compressed. When the goal is a codec for English, there is no fault in them not making a universally bijective codec. (and that's with a bit of handwaving, because there is no such thing as a true universally bijective codec. The pigeonhole principle still applies)