It's not hard to understand that some people might not want to eat it all. That's fine.
It's more that the OP thinks it must be completely impossible that anyone would be able to and is clearly keen to judge those of us who can.
It's fine not to have a massive appetite, lots if people don't - and like you say, some people prefer to graze rather than eat big meals in one sitting. I don't think anyone has a problem with that.
When people talk about 'competitive undereating threads' they mean the ones where people feel the need to be incredibly performative about it, express shock (as the OP did) that others can eat more than them, and use negative terms about eating like 'stuffing', 'scoffing', 'gobbling' etc.
There was a thread recently where someone said that they had some apple slices and hummus for their breakfast every day and that this was a 'nutritious, filling meal' that kept them going until the evening and that they couldn't possibly manage to eat lunch, even though they did 15,000 steps and a Pilates class every day. They were keen to suggest that this meant they had a much healthier attitude to food than people who could manage to eat more than a side salad if they went out for lunch with a friend. That is a great example of Mumsnet competitive undereating.
I have some experience of being around people with eating disorders, and honestly, a lot of the food threads on Mumsnet are full of people who are in denial about their disordered eating, and the language they use around food and the details they feel the need to include are pretty much exactly the same as the sort of language that you hear from people with anorexia and orthorexia. I'm not saying this is the case with the OP at all! But there are certainly people on Mumsnet who are using food threads, consciously or unconsciously, as an outlet for an obsessive/disordered attitude to food intake.