This isn't about files, this is about plain pages of RAM[0]. It is a basic CPU operation to trap on unmapped pages, and OSes rightfully expose this useful feature (in addition to using it themselves), allowing processes to do many things, from lazily-computed memory regions to removing significant amounts of overhead doing a thing the CPU will inevitably do itself anyway.

I believe the "the truncation of memory mapped files" section is for when the Java process memory-maps a file (as Java provides memory-mapping operations in its standard library, and probably also uses them itself), and afterwards some other unrelated process truncates the file, resulting in the OS quietly making (parts of) the mappings inaccessible. Here the process couldn't even check the permissions before reading (never mind how utterly hilariously inefficient that would be, defeating the purpose of memory-mapping) as the mappings could change between the check and subsequent read anyway.

[0]: https://bugs.java.com/bugdatabase/view_bug?bug_id=8327860, "I've managed to narrow this down to this small reproducer:" section