No, _Bool is not a single-bit data type, just as an enum with two values is not a single-bit data type.

When used as a bit field, _Bool is a single bit large.

When not used as a bit field, the standard only guarantees that _Bool is large enough to be able to store the values 0 and 1, and that you can assign integer values that will be converted into the [0,1] range.

But a _Bool can still be allocated a full byte by the compiler, or maybe even a larger type.

The standard added _Bool as a data type for representing logic states. Not as a data type for storage size. The compiler can map a _Bool into a bit, but isn't required to (unless _Bool is used as a bit field).

The standard also says that with the exception of bit fields, all other data types consumes one or more bytes.