BA RTW agents aren't likely to know which flights would earn more Avios though, they're not necessarily Executive Club trained, but as far as I understand it, BA are paid by CX or JL to issue the Avios. If that's the way it works, BA would have earned more from JL than they would have from CX...
Still odd that BA RTW agents are preferring to book prime flights over codeshares, even on flights that don't have a BA code.