A comforter will pretty well fill the tub....don't add anything else...but you might add a couple of tennis balls to the dryer to keep it fluffed.
I used to have a pair of well worn and washed toddler tennis shoes that I used to fluff comforters and down jackets etc in the dryer. They worked very will....but somehow got lost.

We always wash comforters alone. They usually do not have any issues with balancing or spinning because it is one uniform item that is fairly well distributed anyways.

What the manual is getting as is with a large towel or potentially a blanket that will have issues balancing when washing alone. I have a relative that loved to wash her robe separately, but her FL didn't spin when she tried to.

@doreycrouse ah I see. So I'll know if a blanket needs another item if the washer refuses to spin? Like the washer wouldn't attempt to fully spin a load that would damage it?

A single large item such as a blanket or a one heavy item such as jeans or a bath towel are difficult for the machine to balance for spin. It can't tear jeans or a towel in half to arrange the parts on opposite sides of the drum for balance. A blanket, particularly a quilted/cotton type that gets heavy from with water absorption, may tend to plop into a wad. The distribution routine will repeat multiple times to the point that either the machines gives up and won't spin at all or it considerably reduces the speed.

A comforter is typically considerably larger and fluffier than a blanket, maybe synthetic material, and pretty much fills the entire drum space so isn't as much of a problem.

Comforter alone unless it really gets heavy with absorption/doesn't fill up the whole drum. In my parent's LG some of the smaller comforters are finicky with balance, so I'd usually have to add a towel or something to act as a counterweight, but the bigger comforters usually do fine alone.