Regarding your question, you may refer to Adi-lila, Chapter 6, Text 79 along with the purport, (Caitanya-caritamrta). The answer is there. The basic understanding is that Sada-Siva is an expansion of Maha-Visnu and that same Sada-Siva incarnates as Advaitacarya. Therefore, since Advaitacarya is an incarnation of Sada-Siva who is non-different from Maha-Visnu, we can say that Advaitacarya is an incarnation of Maha-Visnu. It can be said both ways, that He is an incarnation of Sada-Siva or Maha-Visnu.
Letter to Mr. Karl, 6 April, 1975