Do sticky bits in Linux permissions make a difference if a group owns the directory

Here is the question, image

Sticky bit is the correct answer, however definitions for sticky bit are:

When a directory’s sticky bit is set, the filesystem treats the files
in such directories in a special way so only the file’s owner, the
directory’s owner, or root user can rename or delete the file.

With directory owner being the main point here.

Since both users are part of the group “IT” which owns the “/projects” directory, why does sticky bit still work here? Peter is in fact unable to delete any files made by Stewie and vice versa despite both being part of a group that owns the directory. Is this a definition error or my misunderstanding?