«C'était tellement amusant qu'il y est retourné plus tard.It was such fun that he went back later! Note that in each case where être is the auxilliary, the verb retourner is followed by a preposition (en, sur, dans, à etc.).
So, in these cases retourner is usually about going back somewhere, or returning somewhere.»
I think the explanation needs a little expansion - considering it is contradicted by the example immediately preceding it.