Many of Samsung's phones, including its flagship Galaxy S and Note series, ship with different processors depending on where they are sold. Some models have the company's own Exynos chips, while others have Snapdragon processors. Almost one year after the phone's release, the TWRP custom recovery now has official builds for the Snapdragon Galaxy S9/S9+.
The recovery image is specifically for the SM-G9600 (S9) and SM-G9650 (S9+) models, which are sold in Hong Kong and Latin America. Those of you with a United States S9 are still out of luck, since Samsung doesn't allow bootloaders on those phones to be unlocked.