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Not all reserved names are reserved for all purposes. _t is reserved only for type names (typedefs), whereas _Atomic and _Bool are keywords.

The standard reserves several classes of identifiers, "_t" suffix [edit: with also "int"/"uint" prefix] is just one of several rules. Another rule is "All identifiers that begin with an underscore followed by a capital letter or by another underscore" (and also "All external identifiers that begin with an underscore").

That only because bool was usually an old alias to int.. It's defined as alias to _Bool in stdbool.h, highly recommended.