The World supertype is available in the supertype dropdown, you know...

But it's Enchant world, not world enchant?

All World Enchantments were printed as Enchant World, but to make them work in modern rules you'll find that they're all World Enchantment in Oracle.

Yep. See the Oracle wording for Field of Dreams or any other former Enchant World.

I'm really confused by this, but maybe it's just me.

So, once again everything MtG has done in the last decade is evil and wrong. Where's the surprise? :)

Ok - as for what this is meant to do, it's "Magic combat the way newbies attempt to play it", which is not co-incidentally the way yu-gi-oh does it. (Flavour text quote coming from http://www.yugiohabridged.com/ )

Instead of attacking a player, you may choose to attack their creatures. "I attack your angel with my grizzly bears and my hill giant so it dies!" is a thing I have had time and again to explain that they can't do.

Actually, thinking about it, this might be wordable just as "All creatures are planeswalkers", but the caveats to let them be cretures too would probably be just as complicated.

Creatures may be attacked as though they are Planeswalkers?

..."All creatures have provoke?"

Attacking creatures like planeswalkers or players is notoriously confusing: can an attacked creature block?

Provoke is a funny suggestion :)

How about "If the defending player controls any creatures, attacking creatures combat deal damage to one of them instead of to him or her."

Or "Creatures can't attack. Instead creatures have ': ~ and target creature deal damage equal to their power to each other.'"?

We're trying to make it work like Yu-Gi-Oh? "Attacking players choose all blockers. Creatures cannot attack players unless the defending player controls no creatures or the attacking creature is unblockable."

It's more "Trying to emulate the confusion of a new player" who typically wants both the "I attack that" and the "I block that" without realising the confusion that having both abilities will cause.