Can you mention your source link ???? Did you see a Buquebus Terminal in Punta Del Este ?
Regarding Punta del Este: The Bradt Travel Guide to Uruguay from 2010 (by Tim Burford) says the following: "Buquebus...runs both fast ferries and traditional slower ships from Buenos Aires; these no longer go directly to Punta del Este, but connecting buses go there from both Colonia and Montevideo." (emphasis mine)
As for Piriapolis, if you look at the Buquebus website, you'll see that place alongside Colonia and Montevideo on the list of Uruguayan terminals. See here: http://www.buquebus....ales_A_pol.html