I think Jim has held off on specifically stating that because doing so would basically confirm Harry survives the series. It's not necessary for a first person perspective for it to have been a journal entry...
Survival isn't necessary for it to be journal entries, either. A journal is a different thing than a
post facto autobiography.
The last page -- or last chapter -- of the BAT might be Molly's account of how Harry died heroically, creating some Great Working at the Outer Gates that bind the Outsiders eternally outside.
Or... y'know... until the next Millennium rolls up, or somesuch...