Nobody is suggesting OP consider ADHD because of her child not being entranced by TV. They are saying it because of the combination of her finding him hard, full on, hyperactive, never stopping, argumentative, impossible to tire out as he just gets more energetic etc. (And yes also not enjoying sitting and doing something quietly, but not that on its own).
Perfectly reasonable thing to suggest keeping an eye on, considering it's very common (likely around 3-5% population, so 1 in every 20-30 children) and he has about half the diagnostic criteria described in this thread.
Obviously nobody has actually made a diagnosis, it's just a suggestion. NHS won't assess at nursery age anyway because there is too much overlap with normal age appropriate activity, short attention span etc.
Since nobody has suggested anything outlandish like dosing him up with coffee, the suggestion is mainly to look at resources about what helps with ADHD type hyperactive behaviour, since at this age most of it will be transferable to any child with a higher energy range, and will be far more useful than generic, frustrating advice OP is finding unhelpful like "put the TV on and give him a snack" or "praise for good behaviour" or "tire him out on the way home".
He might well grow out of it but maybe the info for more extreme situations will help in the meantime.