The companion ticket offer only applies to Club and First ... and only to the full-fare tickets (sometimes full-fare tickets means different things, for example with upgrades for miles they are available on a few fares that are not quite full-fare, but in this case, it really means full-fare tickets).

Without many discounts for Club/First tickets originating in the US, it can be useful. (But there are cheaper deals for advance purchase flights originating outside of the US.)

From the east coast of the US to London or most of Europe, you're talking around 8K for Club, or 12K for First using the companion ticket offer.