Place the rack in the Instant Pot Multicooker, add 1 inch of water, press STEAM, and adjust the timer to 20 minutes.
Once the water is boiling, place the chicken on the rack, cover with the lid, and steam until the chicken is cooked through, 10 to 12 minutes. Press CANCEL.
Transfer the chicken to a plate and let rest until cool enough to handle. Shred the chicken into large pieces.
While the chicken cools, make the celery slaw. In a medium bowl, whisk together the sour cream, vinegar, salt, and black pepper. Stir in the blue cheese. Toss with the celery and scallions, then fold in the lettuce.
Carefully remove the rack and rinse out the pot. Add the ketchup, buffalo wing sauce, and water to the clean pot, and mix to combine. Press SEAR/SAUTE and bring to a simmer. Press CANCEL.
Add the shredded chicken to the pot and toss in the sauce to coat.
Pile the buffalo chicken on the bottom half of the rolls, then top each sandwich with the celery slaw and roll tops.