if my memory serves me well is that in place updates was something MMAP could do.
The little I know about wiredTiger is that data is written compressed on disk. I do not know a compression algorithm that can compress effectively a simple value and replace it in a compressed block. So I think the whole document, non just the array, is written back. something like copy-on-write is implemented.
@MaBeuLux88_xxx, @Jason_Tran, @chris, @wan, can someone point us to some documentation to settle this out.