That study kind of proves that economics is not a science. Not even the "true" value they want to compare their forecast with is correct. It's also just an estimate. Complete nonsense.
"To assess the quality of the forecasts, they are
compared with the actual GDP figures. Since GDP is
normally revised several times, it is necessary to
decide which figure to take as the outcome. Following the literature, we use the first available estimate
for real GDP growth. In our case, this is the annual
average calculated by the State Secretariat for Economic Affairs (seco) in March of each year on the
basis of its quarterly estimates.2"
The main problem is that GDP can and is defined in so many different ways and massaged and changed to anyones liking.
Same with inflation. It's hard to measure correctly in the first place and becomes even more useless as it is defined differently at every occasion or when it does not match whatever one tries to achieve.
("50%" might not always be the right way to look at things, but I assume you mean something better than random or previous.)