I have problems to access that site – probably because I’m located in Europe?!
Though, on Chrome I can autofill – at least the username:
Did you check your settings? E.g. with this “short guide”: [Short Guide / Tips] Autofill on Android
PS: So, to also circle back to your first post, title and request at all: there is no such requirement, that autofill only works when both fields – username/email and password – are present.