CLtL2 is not authoritative for Common Lisp. CLtL2 describes a language prior to the ANSI standardization.

Use the Common Lisp HyperSpec, which is derived from the ANSI CL standard: CLHS 4.2.3 Type Specifiers.

NUMBER is an atomic type specifier.

Type specifiers which can be written as a list with subsidiary type information are called compound type specifier and some are compound-only type specifiers.

The glossary says:

atomic type specifier n. a type specifier that is atomic. For every atomic type specifier, x, there is an equivalent compound type specifier with no arguments supplied, (x).

This would indicate that (number) is a valid type specifier. Some implementations accept it: LispWorks, ABCL, GCL, ...

Since the glossary is part of the standard, this might be a bug and/or an omission in SBCL, CLISP, Clozure CL, ...

Then this usage of type specifiers is legal: