Pour Soyaki into a 13” x 9” x 2” baking dish. Add steak; turn to coat. Cover and marinate 1 to 2 hours, turn occasionally. Note: remove steak from refrigerator about 40 minutes before grilling to allow it to “warm up” to room temperature.
Marinate Mini Peppers
Whisk chili sauce, oil, vinegar, honey and Sambal Matah in a small bowl; season with salt and pepper. Place mini peppers in large bowl and toss with 6 tablespoons of dressing. Cover and marinate for 2 hours at room temperature. Set aside remainder of dressing.
Preheat grill to high.
Add steak and grill to desired doneness, about 4 minutes per side for rare. Transfer to work surface, cover and let rest 10 minutes. Thinly slice against the grain, transfer to a bowl and toss with some of remaining salad dressing.
Divide arugula among 4 plates. Toss watermelon and peppers together, then divide the mixutre among the plates; top each with 4-5 strips of steak; drizzle with remainder of dressing and serve.