No, no one is saying Henderson is the best option out there, many just think he is a feasible option. The best keepers may be Oblak, Alisson, Handanovic, Neuer, but they are not easily gettable and they cost a lot, while Henderson is already one of us and we can upgrade without extra cost.
Even an error-prone, average de Gea is capable to be United's no. 1, how come a Henderson with more clean sheets, more saves, fewer goals conceded than expected, fewer errors, less protection from the defence not good enough to be one?