Can somebody tell me how can hyperbaric oxygen therapy (hbot) treat diabetic foot ulcers?

By definition. all wounds are ischemic. meaning they don't have enough oxygen for healing. HBOT super-saturates the wound increasing the chance for healing.
Oxygen concentration. Under pressure, the oxygen concentration in the serum and adjacent tissues increase allowing for a temporary improved oxygen delivery to the wound.
Diabetic Foot ulcers. Hyperbaric oxygen can be used as an adjunctive therapy for diabetic foot ulcers under two main conditions: when the ulceration is deep to the muscle, tendon and bone with signs of infection; and or if there is a bone infection that is chronic and refractory to other treatments.