For smaller economies modelling some months into the future is actually pretty decently possible. Of course, there could always be big events throwing everything in a new state.
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.
Do you seriously mean that smaller economies can be modelled several months into the future with accuracy and be correct more than 50% of the time?