It's the latter. All those numbers are estimates based on what we’ve seen with the interval between doses in the two-dose studies. No studies have been specifically done on efficacy of a single dose; not for Pfizer/BioNTech, not for Moderna, not for AstraZeneca/Oxford. However, that limited trial data suggests moderate to high efficacy after a single dose; the main issue seems to be lack of data on how long does the immunity last, whether it fades away much faster without the second booster dose, and whether a longer delay between the two doses is significantly detrimental for long-term immunity. More data would be nice, but either way, the limited data we do have seems to suggest that a single dose is enough to prevent severe cases and significantly reduce hospitalisations for weeks (longest interval between the two doses was 26 weeks in the AZ/Oxford trials), which is pretty encouraging.
The only vaccine that has trialed a one-dose protocol is the Johnson & Johnson one, the data should be available any time now, and they are also running an additional two-dose protocol trials, so that should be very useful in providing a direct comparison of a single vs multiple dose regimens.