A lot of the plants he is growing actually grow in what is considered an everwet rainforest environment where the soil doesn't drain too well, they are almost bog plants but not quite (a lot of rainforest floor plants in Asia are like this), so humidity would have a big part in that in keeping the soil moist, but aside from that the pots being used are net pots with hydroton and soil in them so drainage won't be a concern unless you use a mix that retains moisture which means it really doesn't matter if it's sloped anyways. A lot depends on the type of plants you want to grow I see a lot of people growing epiphytes on the ground which changes the goals for the soil and moisture retention of the enclosure, I've rambled enough but soil composition is going to have way more impact on drainage than holes in the tray will..
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