My guess is that the rules you quoted apply to standard roundtrip awards, not to the multi-partner awards. AFAIK, you can get eight segments on a multi-partner award even without any segments on BA, and such awards can be used for one-way travel, too.

In fact, do the quoted rules pre-date the conversion of BAEC from a zone-based system to a distance based system? Now that BAEC charges based on distance flown (by segment on "regular" awards, and by total distance on multi-partner awards), it's hard to see why BAEC would care whether you flew the most direct route. To the contrary, the more indirect the route, the more Avios BAEC can charge you under the current scheme.