In the documentation at one place is written:

aria_pagecache_buffer_size: The size of the buffer used for index blocks for Aria tables.
https://mariadb.com/kb/en/aria-system-variables/#aria_pagecache_buffer_size

In another location:
Page format is the default format for Aria tables.
It's cached by the page cache, ...
https://mariadb.com/kb/en/aria-storage-formats/

and:

aria-pagecache-buffer-size
This is where all index and data pages are cached.

So it looks like the first source is wrong. And this is the most important one where everybody searches first...