The worst way to store and manipulate text is to use an array. Firstly, the entire file must be loaded into the array first, which raises issues with time and memory. Even worse still, every insertion and deletion requires each element in the array to be moved. There are more downsides, but already this method is clearly not practical. The array can be dismissed as an option rather quickly.
Someone has clearly not heard of Notepad nor used the prolific quantity of text editors for DOS that appeared in the 80s/early 90s, a time when memory bandwidths were in the low MB/s; or even before that, micros with CP/M where memory bandwidths were measured in KB/s (and the whole address space was only 64k). Modern systems have memory bandwidths of dozens of GB/s.
Or perhaps that old saying really applies: worse is better.
IMHO an array is all you need for a regular general-purpose text editor. I hate needless complexity in software, but unfortunately others seem to love it.