Technically it may not be a copyright infringement - whether emojis can be copyrighted is quite complicated.
However, this is irrelevant as App Store Review guidelines say the following.
> Apps may use Unicode characters that render as Apple emoji in their app and app metadata. Apple emoji may not be used on other platforms or embedded directly in your app binary.
The problem here is that generally this is decide by courts, not by apple.
This is basically yet another piece of evidence of a monopoly. Apple acting as the judge, jury, and executioner regarding what it thinks is “it’s rights”
However, this is irrelevant as App Store Review guidelines say the following.
> Apps may use Unicode characters that render as Apple emoji in their app and app metadata. Apple emoji may not be used on other platforms or embedded directly in your app binary.