I checked many sites, all followed the same procedure, seems a standard repair for most size holes, the shell may repair itself in time and growth of the shell is very slow, this is one of the biggest tortoises I have seen here in Turkey and can assume it is nearly fully grown anyway.
The repair is done in small patchwork and there is no shell to breathe under most of it as it was a big hole, it certainly would have died soon left open to the elements.