That's not what backward-compatibility means here. IE6 sees ordinary JavaScript code and executes it. It's true that it can't do the typed-array magic in the background, but it doesn't have to. The JavaScript code that IE6 sees has the same result as compiled asm.js code would; it just does it in a less-optimal way.

That's all that asm.js really is: a set of JavaScript coding conventions that (when followed) prove to be easy to optimize to near-native levels of speed, along with a compiler hint to tell the runtime that these conventions are being used. IE6 doesn't understand the compiler hint, but because the asm.js code is still valid JavaScript, it'll still run.

I suspect that either nnethercote and I are using different definitions of backward-compatibility, or nnethercote is mistaken as to what exactly asm.js is. Authorities are less likely to make mistakes, but they can still make them.