Ah, no FPC_HAS_TYPE_EXTENDED is enabled by the compiler if the compiler supports the type.
The documentation tool however doesn't have the info what the compiler defines, so this is used to fake this.
Basically if the constants are not visible, your platform does not have support for extended and extended is an alias for double. (extended is a x87 specific type)
Simply enabling the ifdef is useless since then the compiler doesn't support the range ( exponent 10^4000 something) that the constants have, so it won't compile anyway.