The outer lanes are the left and right lanes. Which implies that, given 3 lanes, someone is passing on the right.
At a base level, given more than 2 lanes, there is a difference between "stay out of the left lane unless passing", and "stay out of the left and right lanes unless passing", as it gives drivers twice as many options for passing.
I would have said that in USA the "outer lane" is the rightmost lane, since it's the one that forms the edge of the highway. Likewise, in UK it would be the leftmost lane. The inner lanes are the ones adjacent to the median in a divided highway. I think OP simply misspoke, because everyone knows one is supposed to pass using the inner lanes.
> Rule 264
You should always drive in the left-hand lane when the road ahead is clear. If you are overtaking a number of slower-moving vehicles, you should return to the left-hand lane as soon as you are safely past. Slow-moving or speed-restricted vehicles should always remain in the left-hand lane of the carriageway unless overtaking. You MUST NOT drive on the hard shoulder except in an emergency or if directed to do so by the police, traffic officers in uniform or by signs.
You should only go into the outer lanes to overtake traffic, and then move back in once you've passed. It's not just the law, it's a good idea...
That's not about passing on the right.