It’s just a matter of inputting the correct data into the Landscape Grass node in your material. The Grass node input is evaluated for your entire landscape and values > 0 will have some grass and the larger the number, maxing out at 1, the more grass that will be placed.
If you want only the purple areas, one idea is to take the red and blue channels of the splat map, add them together and multiply by 0.5. That will give you a number between 0 and 1 as to how purple the current area is. You can connect that to the A input of an “If” node. Connect some high constant like 0.9 to the B node. For the “A < B” connect a constant 0 and for “A > B” and “A >= B” connect a constant 1. Then connect that to your Grass input.
This will place grass only when average of both the red and blue channels in the splat map is greater than 0.9.
There are many other possibilities to avoid vertical grass without using the splat map. Your material could check the World Normal Z component and exclude grass when it’s greater a certain number, for example.