In the end, we came up with a system of feats that allow you to borrow abilities and powers from other classes. At 11th level, you can choose to forgo your paragon path in order to further specialize in a second class. This approach lacks the intuitive elegance of the 3E system, but it allows us to tone down or boost a class's multiclass options as needed. If everything works as planned, you have the flexibility to mix classes without making your character into a juggernaut or a cripple. Combos like fighter/wizard now work much better, while traditional choices like fighter/rogue still function just fine. Going forward, we'll introduce new feats for new classes, ensuring that all classes play well together.
This seems to be saying that you can't multiclass from level one, only levels 11 through 20.
Also, I think the levels that are in the benefits section of the feats table have something to do with this:
When you take one of these power-swap feats, you give up a power of your choice from your primary class and replace it with a power of the same level or lower from the class you have multiclassed in.
Since powers have a specific level that they are available at, I think the feats allow you to choose a power with a maximum level set by the feat; 4th, 8th and 10th respectively.
That is just how I am interpreting the wording