Hi, the water quality in Ireland is very good, in his region the tap water is 100pm so with a 40inch RO unit the ppm is reduced to 1.
With only that Di unit he can produce about 2200L of pure water and with a RO unit before, he can produce about 220 000L of pure water.
If someone use 500L per day 5 days a week he can works about 88 weeks before changing the resin in a 11L vessel
Those calculations are approximative of course.
Anyway I believe the performance he pretends is true if you put a RO unit before.