Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Heat sesame oil in the pot. Add chicken thighs and saute until browned on the first side, 3 to 4 minutes. Add garlic and flip the chicken, sauteing until the other side is browned and the garlic is fragrant, 3 to 4 minutes more. Hit Cancel and transfer the chicken to a plate using tongs or a slotted spoon.
Pour 1/4 cup water into the pot and use a wooden spoon to scrape any browned bits from the bottom. Whisk soy sauce, hoisin sauce, and honey together in a bowl until smooth. Return chicken to the pot and pour sauce over the top.
Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 10 minutes. Allow 6 to 10 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions, about 5 minutes. Release remaining pressure carefully using the quick-release method, about 5 minutes. Unlock and remove the lid. Remove chicken with tongs or a slotted spoon and set aside.
Mix remaining 2 tablespoons water with cornstarch in a small bowl. Turn on Saute function and whisk cornstarch slurry into the sauce. Simmer until the sauce thickens, about 2 minutes. Cancel Saute function. Return chicken to the sauce, sprinkle with green onions and sesame seeds, and toss everything together.