Sounds like Alex is talking about a characteristic of the airfield - i.e. there is a balanced field length available, due to the particular construction of the runway/overrun etc., while the usual way BFL is used is in the context of an aircraft's field performance, when we are really talking about a balanced field length requirement.
It's no different, philosophically, than talking about TODR and TODA, except that for whatever reason, no-one ever appends the "A" or "R" to BFL.
If Alex (and the RAF V bomber crews!) had called their "BFL" "BFLA" and the rest of the world called their version "BFLR" then it'd be much clearer. As it is, saying "balanced field length" can be as confusing as just saying "take off distance" - if it's not obvious from the context, there are two interpretations.