Without reading the definition again my understanding was that minimum fuel declaration meant that you had committed to one airfield for landing and that any further changes to clearance meant that you might go into final reserve. This could be possible for the en-route scenario whereby the crew after a long re routing might have been able to obtain destination weather, and perhaps with knowing it has more than one runway available decided to commit to it and use their diversion fuel to get there, thus putting them potentially into a minimum fuel state.