Yeah, they say that, and yet they don't want explicit tailcalls either.

iirc JSC folks are blocking explicit tailcalls on TC39

Well, not surprised, because they actually did the work and implemented the spec calling for implicit tailcalls, and then it turned out that the other JS engines were to lazy or biased to implement it themselves (mind, all the other parts of the spec were fine, but the one thing they disagreed with they just didn't implement).

So yeah, I'm sure I'd be pretty pissed if I were the JSC devs, and not so happy about the community turning around and saying now we want explicit tailcalls. And by the way, they're also probably smart enough to see that the V8, SM, and ChakraCore are probably not really going to follow through with explicit tailcalls anymore than they would with implicit ones.

Being explicit won't make the implementation any easier, it will just firewall off any performance issues (note that JSC achieved this with implict tailcalls). And since the other engines having been complaining against how hard tailcalls would be to implement, it's unlikely they'll do it regardless of whether they're implicit or explicit.