a reasonably sophistocated attacker would simply get the diary into a text file then search for strings of keywords in the bip 39 wordlist. "blanket air setup scrap feature actress prison bitter cash torch vacant valley" would stick out like a sore thumb. so keeping the 12 words together would be like handing over your seed phrase to an attacker on a silver platter.
It indeed won't help against a targeted attack, but how likely are those? 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.