Have you checked their oven and hob to make sure they're working?
Has there been an incident they didnt mention to you involving a pot or pan left on the hob far too long, or even a minor fire incident? Has one of them dropped a full pot? (Older peoole dont have the same power in hands and arms that they may have had earlier in life).
Are they able to manage washing up and putting away of pots and plates after cooking a meal? Have they been ill from eating leftovers that were left out too long?
At their age, it might be a good idea to have their teeth checked. If they have false teeth, they may need to get them re-fitted, as gums shrink along with the rest of the body as people age. Also, a few years of eating a diet of fairly non nutritious food (snack foods, etc) can result in cavities, making eating painful.
My DM is in her early 90s and has never eaten more than a sparrow on a daily basis as long as I've known her. She lives on home made chicken soup (bones, veg, stock cube, water), eggs, toast, marmalade, porridge, bananas, Greek yogurt, the occasional potato/ sausage/ ham/ chicken / cauliflower/ broccoli meal that she makes herself, the occasional chicken curry with rice and occasional roast dinner delivered from a nearby gastro pub (one adult size plate lasts her three days), digestives, cheese, an occasional bite of ice cream, occasional toddler Hipp meal, and protein shakes which are doctor prescribed for supplemental nutrition. Also several pots of tea per day. It has been an enormous struggle to get her to drink water, which she never did when she was younger. Probably about 900 calories a day - her servings are tiny.
She uses the hob, doesn't trust the microwave except for warming up tea, and her oven isn't working right but she won't get it fixed or replaced.
Supplements - vitamin D and vitamin B12.