From the paper page 6: “Holding all else constant, increasing the portfolio weight for Swiss equities exposes an investor to higher volatility than that for the entire market. Therefore, a Swiss overweight implies embracing additional volatility without necessarily gaining any expected return.” Have we read the same paper?
Correlations with MSCI world has increased over the time which means that an overweight of Swiss stocks has less impact than in the past.
Between 2009 -2020 the correlation was 0.93 https://www.portfoliovisualizer.com/asset-correlations?s=y&symbols=EWL+VT&timePeriod=4&tradingDays=60&months=12
You also need to take into account the other advantages of a global portfolio: company diversification, sector diversification