The reference samples were taken from Population Reference Sample (POPRES),  Human Genome Diversity Project and  Leipzig sample (LPZ) Veeramah et al. (p. 996).
POPRES sample was described in another article 'Genes mirror Geography within Europe' (2008) Novembre et al. I am attaching documents for 'Genes mirror Geography within Europe' by Novembre et al. including supplementary and 'Genetic variation in the Sorbs of eastern Germany' by Veeramah et al.

Discussion in 'Supplementary for Genes mirror Geography within Europe', Novembre et al.

In addition, there were several small samples in PROPES project

Then, authors used a bootstrapping a technique often employed by statisticians to address the problem of small size samples inducing average results based on a probability distribution.

The conclusion in 'Genes mirror geography within Europe' Novembre et al. study

Veeramah et al. in 'Genetic variation in the Sorbs of eastern Germany' produced 4 PCA plots in total (p. 997)

-Two PCA plots based on PCA analysis with and without POPRES/LPZ merge
-Two PCA plots using a bootstrapping technique with and without POPRES/HGDP merge

The two PCA plot I put above are based on a bootstrapping technique as in the other two plots it’s difficult to make out between ethnicities on the plots The four plots are similar anyway as mentioned by Novembre et al.

The obvious relevant outliers are Slovakian individual located in the bottom right corner and Russian individual. I would assume the plot you obtained from 23andme site has also come from 'Genes mirror Geography within Europe' by Novembre et al. study, as it’s been utilised by other researchers.
In fact, the article 'Genes mirror Geography within Europe' by Novembre et al. from which PCA plots were taken was cited around 285 times in other published scientific studies. So, it proved to be a popular scientific study and we may find the same reference European populations being used in other studies.  However, the abstract and the plots of the study I put above was specific to Lusatian Sorbs as they were sampled seperately. Any conclusion in regards to genetic proximity between other ethnicities on PCA plots should be taken with caution.

The genes mirror geography is the same map that was used in the Sorb study just displayed somewhat differently. As can clearly be seen by the fact that the locations of all of the countries are the same.

First of all care to explain what does YG- or Yugoslavia concretely represent? This study is from 2008 yet Yugoslavia broke apart as a country in the early 1990s- more than a decade ago since the study has been published.

Secondly a "Yugoslav" could be anything from an albo in Kosovo to a Slovenian.

Thirdly if Yugoslavia represents Serbs only then what does RS represent? Since RS is the official country code of Serbia, like RU is for Russia, PL is for Poland, SL is for Slovenia, HR is for Croatia etc….. Yugoslavia at the time this study was carried out did not exist.

Finally why hasn't any other DNA test ie Polako, Dodecad, Macdonald etc…. ever found an ethnic Serb to cluster with Greeks and Albos? According to this study one would expect that about 1/3 or so of Serbs would fall into the Greek cluster.