I don't MacKays estimates but the cited study seems to assume a capacity factor of 50% for a 15 MW turbine.

Average turbine capacity of turbines installed in 2021 was 7.4 MW but all manufacturers seem to have 15+MW designs in the work [1]. So the projection is slightly optimistic IMO. The capacity factor seems slightly more optimistic at 50% as worldwide average seem to have fluctuated between 35 and 45% [2]. Although other numbers are closer to 60% for the UK [3]

[1] https://www.energy.gov/eere/wind/articles/offshore-wind-mark... [2] https://www.statista.com/statistics/1368679/global-offshore-... [3] https://windeurope.org/about-wind/daily-wind/capacity-factor...