In a small mixing bowl, whisk together the coconut aminos, ketchup, rice vinegar, fish sauce, sesame oil, tapioca starch, and chicken broth until combined. Set aside.
Heat the oil in a small saucepan set over medium heat. Once hot, add the garlic, ginger, and red pepper flakes. Cook, stirring, for about 1 minute, being careful not to burn.
Add the coconut aminos mixture and stir to combine. Cook, stirring often, until the sauce thickens, about 4 minutes. Remove from the heat and set aside to cool.
In a large bowl, add the chicken, salt, and pepper. Once the sauce has cooled, add 1/4 cup to the chicken, cover, and marinate in the refrigerator for at least 1 hour or up to overnight, reserving the remaining sauce for serving.