Hi Gysbreght,
if there is a PAPI/VASIS on a downsloping runway, would it be located 1000 ft from the threshold, pointing up at 3 degrees to the horizon, thus providing less than 50 ft threshold crossing height?
If you look at the ALC link I posted #23, the runway has a down slope of 98 feet in 3,000m (1%). The ILS glide slope transmitter is located 350m from runway threshold (difference between LDA (3,000m) and "distance beyond glide slope" (2,650m)).
The 3 deg glide slope would be 18.3m (60 feet) above the threshold on a level runway. Alicante's runway threshold is 11 feet higher than the glide slope transmitter, so the threshold crossing height is 49 feet.
The published LDA (no inset landing threshold) is 3,000 m, but the effective landing distance is 50m shorter (due to the down slope).
The converse would be true if you landed on the reciprocal runway and crossed the threshold at 50 feet.
Maybe that was the point the OP's Captain was trying to make?