Extensive green roof on a flat roof or with a 2% slope on the garage?

So the roofer really went full throttle just now and desperately wants to persuade me to go for a zero roof slope with two inlet boxes. He is very convinced that otherwise, over the years, the sediment components and everything will gradually move down the slope, and he finds a parapet on all 4 sides without a gutter visually (like us) simply much prettier. He has a hard time not offering it as a 0% slope. I think he is trying to change my mind, especially since the implementation is much easier.

Ok, there will now be a 1.5% slope with two inlets and a surrounding parapet. No channel. :-)

I don't know if it helps you, but I can tell you what the site manager garage, landscaper, and construction consultant told me. It is also about our garage roof, but this one is supposed to get an intensive green roof. With 50 cm substrate/soil for lawn and hedge. I also wanted a slope because of moisture, etc. Everyone said no, garage roof is waterproof anyway and the water on the roof must not constantly drain completely because then nothing can grow. Now there is zero slope, parapet all around (not visible on one side because it is very low) and the drain is installed so that about 5 cm of water is retained for the plants.... Best regards GD