V8 is about 100 times (seriously) faster than IE6's JS engine. So, yes, "backward compatible with IE6" doesn't really mean anything.
After executing 5 million statements, IE will ask you if you want to continue. It will ask you many times. You'll probably sit there all day long clicking the whatever button.
When the 'competition' is being advertised as an easy migration path for old software, then "backward compatible with IE6" means everything - it's just another way of measuring how much pain is involved in migrating some software over, only here instead of having to switch your entire corp and customers to Chrome, the change is mostly restricted to the developer toolchain.
You're continuing to perpetuate the notion that asm.js only has implications for high performance software, despite replying to a comment where I gave you a very real use case that has nothing to do with performance. And as I suspect you already know, since you seem knowledgeable, GC can easily be implemented in about a million different ways using an emulated heap on top of a typed array, so yes, even a naive implementation of VB4 could easily be made to work.
It doesn't make IE6 any faster, but the code will still run. I suspect that's what they mean by "backward compatible": it can't work miracles, but will at least do no harm.
"Run". Yes, just ~600 times slower. At that speed it probably won't be very useful. You have ~50 msec for an operation which is perceived as instantaneous. If it takes 30 seconds (600 times as long), people will not use it. It simply isn't feasible.
This stuff is only ES3 compatible because it didn't need anything from ES5. A low-level compiler target doesn't need any of these things: http://kangax.github.com/es5-compat-table/
After executing 5 million statements, IE will ask you if you want to continue. It will ask you many times. You'll probably sit there all day long clicking the whatever button.