Comparison of basic strengths of hydroxides of alkali metal group
Yes that is it. 

To paraphrase the wikipedia article HSAB theory

"Hard and soft (Lewis) acids and bases" (HSAB), also known as the Pearson acid-base concept, HSAB is widely used in chemistry. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 


'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 
'Soft' applies to species which are big, have low charge states and are strongly polarizable. The concept is a way of applying the notion of orbital overlap to specific chemical cases.


Pearson evidently had revised values in the paper (which is behind a firewall):


Pearson, Ralph G. "Absolute Electronegativity and Hardness: Application to Inorganic Chemistry." Inorganic Chemistry, volume 27, number 4, 1988, pp 734–740. https://doi.org/10.1021/ic00277a030


The data from that paper is on this webpage: http://www.knowledgedoor.com/2/elements_handbook/chemical_hardness.html

\begin{array}{|c|c|} \hline
Ion &amp; Chemical\ Hardness\ (eV) \\ \hline
\ce{Li+} &amp; 2.39\\ \hline
\ce{Na+} &amp;   2.30   \\ \hline
\ce{K+} &amp;  1.92    \\ \hline
\ce{Rb+} &amp; 1.85     \\ \hline
\ce{Cs+} &amp;   1.71   \\ \hline
\end{array}