Nope, HG is not a bitwise. That's the LF and (sorta) the HNG.
You do indeed have extra pages due to the B tree, that's why it takes up so much space compared to the other types. But because of the G-array for duplicates, the B-tree only indexes the unique/distinct values in the column, so it's not as expensive as a traditional B-tree. Also, the B-tree leaves only have to be large enough to fit a pointer back to the FP, they don't store the actual data, so that's another space saving.
