I would guess that Oxford people know best about the sort of work they have done and the possible obstacles. All of the others (not directly familiar with the work done) are merely speculating and making educated guesses based on what they think is true regarding the virus.
Once again, I am interested in knowing how can a rando on the internet be certain that there will not be a vaccine ready in 2+ years if there is a team of scientists thinking that it will be in six months? Ok, it genuinely being six months would be too-good-to-be-true news, but how arrogant must you be to write it off outright?
At the same time, we are unsure even regarding basic things, such as the death rate and if one can get reinfected. This is also another reason why opinions difer - no hard data means more room for speculation.