In the chatty readme is written own server needs to support XEP-0313: Message Archive Management among others. AFAIK libpurple still doesn’t have mam support so i presume it’s a kind of typo?
Neither I found mam implementation in chatty itself (only receipts).
UPD: ok I found several clues in the code that it’s on todo, also finally learned to use gitlab and found relevant issue record. So seems todo is not moving fast, will see if i can put some help.