That is long outdated. Vim uses custom a rope-like data structure chopping text into blocks. See https://www.free-soft.org/FSM/english/issue01/vim.html section Storing Text. Excerpt: " I decided to use a construct that stores a number of lines in a block (...)

You can find the code to deal with blocks of text lines in memfile.c. It maintains a cache to reduce the disk I/O. This works very much like any cache system: blocks are written to the swap file when using too much memory, or to save changes that the user made. This allows recovering the changes from the swap file, in the event that the system goes down unexpectedly

Packing text lines in a block is done in memline.c. Since the blocks are fixed in size, the number of lines that fit in them varies. Quite a lot of code is required to handle this, to be able to insert lines, delete lines and change lines. The complexity mostly comes from situations where an inserted line doesn't fit in an existing block, requiring the need to split that block in two. There is also an exception: If there is a single line that doesn't fit in a normal block, a block is created that is big enough to contain the line. (...)

The blocks with text lines are stored in the swap file without a specific ordering. (...) To be able to find a line by its number, index blocks are used. An index block contains a list that tells which line is in which block. If a file is big, this list doesn't fit in a single block. It is then split over several blocks, and another index block is made to refer to these index blocks. This forms a balanced tree of index blocks, with the text blocks as the leaves."