Today I met someone who had no idea that password managers existed yet had realised the need for unique and complex passwords so was generating and storing them all in an excel spreadsheet.
So close to the solution yet so far.
Discussion
If the spreadsheet was encrypted and store on removable media, then that would be fairly secure. But I'm guessing it was on the hard drive in clear text.
Your guess was correct.
Still perhaps slightly better than another friend who I found had all his passwords saved in his iCloud synced iPhone notes.