This sounds like a myth perpetuated by strong folk who have had a hard time on slabs with supposedly moderate grades.
In their defence, I have a hard time differentiating the difficulty of most Font slabs. Forêt Noire (7A+) feels as hard to me as Super Prestat (7B+).
Some problems in Font do seem have a bit of danger money attached to the grade, but it's not entirely clear how much, and doesn't seem to apply to old school terrifiers like Le Pilier Légendaire at Éléphant, which is E5 6b but was 6C back in the day.