Is grep part of coreutils? (If not, why?)

The authoritative source would be http://www.gnu.org/software/software.html from the Free Software Foundation rather than the Wiki page cited in the accepted answer. On that page grep is listed as a separate piece of software.

To quote the Arch Wiki page you refer to:

The scope of this article includes - but is not limited to - those utilities included with the GNU coreutils package.

So even though the article covers grep, this doesn't automatically mean it's part of coreutils.

Moreover, this article doesn't list grep among the tools in coreutils.