I think the following sentence is wrong or could be misinterpreted-
If you use an expression in the GROUP BY clause the same exact expression must be used in the SELECT list.
I think it means that a column used in an expression in the GROUP BY can't be in the SELECT list unless the expression itself is used.
My point is that it doesn't have to be in the SELECT list.
for example, SELECT MAX(SalesDateTime)
FROM dbo.SalesTransaction
GROUP BY CONVERT(CHAR(10),SalesDateTime,101);
or SELECT COUNT(*)
FROM dbo.SalesTransaction
GROUP BY CONVERT(CHAR(10),SalesDateTime,101);