Some technicalities :
When aiming the pilot eye (or ILS antenna) on a 3° slope to the 300m marker, the gear will go over the threshold at a significantly lower height.
For a medium size jet, you're about 4.5m high, to which you add a 3 to 5° pitch angle times 15.5m, that's about one more meter.
So your gear will be above threshold at around 9.5m and would impact the runway at about 190-200m if you did nothing.
However, any flare even a very late (but successful one) will bring you well over the 300m mark. Usually a very good landing is done at 400m. Anything before that can usually be hard (assuming a correct height over the threshold obviously).
So I don't understand how any operator could guarantee 300m landings everytime.
Maybe they plan to approach lower : "Approach guidance is consistent with a shorter air distance"
Which wouldn't shock me as long as there was a sufficiently long displaced THR or similar clear area before it. But it isn't allowed at my operator, we land between 1000 and 2000ft (some similar operators will allow up to 3000ft)
However, at least for airbus aircraft, I noticed the flysmart perf app is very pessimistic when it comes to landing distance.