When he was underfueling, he was only eating 1800-2000 calories a day, which also contributed to the stress fracture that prevented him from going to Doha last year despite qualifying.
2500/day is still extremely low. Another runner who doesn't eat much is Evan Jager - weareeaton.com has a 2-week routine of his nutrition. Jager also seems to be around 2500/day, at 140 lbs (which is skinny for a 6'2" guy).
I'm at 5000-6000/day (160 lbs), if I eat less I would be hungry and also increase my risk for stress fractures. Jordan Hasay was self-reported to eat 5000/day, which seems more believable/realistic for a pro runner.
It is very individual and depends on muscle fibers - ST fibers need a lot less than FT fibers, so distance runners like Hunter and Jager who have 70% ST and phenomenal aerobic abilities don't need to eat as much as sprinters or people like Rudisha, also they have less muscle mass which also requires less calories. Maybe 2500 is the right amount for Hunter - it probably is - maybe it's not. Keep in mind he is burning more fat relatively and much more efficient at it than us recreational 15-18 min runners.