MT is too old and plagued by ill-health. That is why he doesn't write scripts/screenplays anymore. The last proper one he wrote was PazhassiRaja (9 years back) which to me was one among his weakest scripts, irrespective of how well the movie was made. There is no depth or layers in the screenplay/dialogues at all. That should be largely be because of the failing health. He has spoken about that in one of the interviews maybe 5-6 years back. It is not physically possible for him to sit for long hours and write a script. The physical condition will also affect the thought process. Even movies like Ezham varavu and Neelathamara are remade from his old scripts - not new ones. Even if you take last 20 years, we can easily see what MT has written for movies - Daya, Ennu swnatham Janakikutty are extension of his short stories and simple ones which needed less efforts from him. The last deep one was Sukrutham way back in 1994. He did adapt another writer's story and directed the beautiful 'Oru Cheru Punchiri' in between though.
So Randamoozham screenplay is almost definitely not written by MT. However, I do not believe that someone cannot make a MTyish monumental script out of Randamoozham because the basic story/perspective is out there. Also, the screenplay may not matter much. The technical grandeur possible these days can easily make a movie big hit / immensely watchable. But if you really need a deep, soulful, intensive adaptation then it will be difficult. I dont think that is the makers' aim though, especially considering the budget. The makers may be hoping for a Bahubalistic protrayal plus any scripting brilliance they can scrap out of Randamoozham - not a bad hope by any means