Is C++11's long long really at least 64 bits?

The C++ standard references the C standard for this, see [c.limits]. LLONG_MAX is defined in <climits> with the same meaning as C's <limits.h>'s LLONG_MAX. And this is what C has to say:

5.2.4.2.1 Sizes of integer types <limits.h>

The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives. [...] Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign.

[...]

-- maximum value for an object of type long long int

LLONG_MAX +9223372036854775807 // 2⁶³ -1

A signed type that must be capable of representing the value 9223372036854775807 requires 64 bits or more.