Why is Phobos' isIntegral different from the built-in trait of the very same name?

[Andrei Alexandrescu]

According to https://github.com/dlang/phobos/blob/master/std/traits.d#L6191:

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Detect whether `T` is a built-in integral type. Types `bool`, `char`, 
`wchar`, and `dchar` are not considered integral.
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Not considered by whom? Certainly they are considered by the language 
per __traits(isIntegral). Also all conversion rules scream "they're 
integral!"

We even use the built-in trait to create a _different_ one!

Why do we have a slightly different with no added usabilty in the 
standard library?