It indeed won't help against a targeted attack, but how likely are those?
if the thief knows about bitcoin, i'd say it is very likely.
I'm talking about the millions of burglaries where the thief looks for valuables and wants to get out as quickly as possible again. 12 words under your pillow will be noticed, 12 words in a long written text will be ignored.
a thief knows that a diary can be used to blackmail someone. if it has really sensitive personal information or just information the person doesn't want anyone to know. that's why a diary is a liability just as much as a bitcoin private key on a piece of paper is a liability if anyone finds it.