I know that Skyteam have a "only guest someone on the same flight into the lounge" policy. This caught me out in RUH on SV having offered to guest someone in that I met on the previous flight. Was a bit embarassing at the lounge

I must say, it does generally make sense. I guess not many people will be travelling with someone they know, at the same time, but on a different flight. If these rumours are true, I guess that more people do this than you'd think AND BA thinks it's costing them too much. Apart from the people that put their details on FT offering to guest, it probably won't effect many (BAEC) people.