Domestic arrivals are always clean, but the LHR inter-terminal connections always force you to go unclean. Given how few domestic arrivals exist to start with that's almost excusable, unlike the UK governments insistence that foreign arrivals are always unclean.
Ah OK, yes, I see what you mean now… You have to take the bus from T5 and then there is security when you arrive at the other terminal, I think. I guess their point would be that you have then mixed with non-clean pax, although that should be LHR’s problem to sort out by separating pax streams.
I think it boils down to how important an airport thinks transfer pax are. The major carrier at LHR always saw (rightly or wrongly) transfer pax as fairly unimportant, so that influences how LHR handles transfers. I guess LH sees them as more integral to their model, so that influences Fraport.