I think they meant the other way around, that if you wanted to use it in C/C++, you'd have to either use assembly or some specific SSE construct otherwise the compiler wouldn't bother.
That probably was the case at one point, but I'd be surprised if it's still the case. Though maybe that's part of the reason why the Intel compiler can generate faster code. But I suspect it's more of a case of better optimization by people who have a better understanding of how it works under the hood, and maybe better utilization of newer instruction set extensions.
SSE has been around for a long time and is present in most (all?) x86 chips these days and I'd be very surprised if gcc and other popular compilers don't use it effectively today. Some of the other extensions might be different though.