He says "they offer us an irreplaceable monastic and Christian experience." Which is great, because it sets his comments strictly in the context of monastic experience, ie people who have devoted their lives to full-time prayer and detachment from the world. Which is not the case for most Christians.
He also notes that
"it is not at all impossible to make such things [meaning Latin and chant] understood and appreciated. Generally I succeed quite well in this, in the novitiate, with some exceptions, naturally, who did not understand well."
So some his novices - highly motivated, aspiring professional-prayers in full time formation with an experienced guide - couldn't manage to understand Latin and chant.
If some of them couldn't manage it, then what hope for the average Christian whose capacity for such formation is intrinsically less, simply because they have to devote the majority of their time to the affairs of the world.
My conclusion: Chant is great for people who have the gift of praying with chant. But that isn't all - or even most - of us.