Here Goss talks about being active defined as posting at all during a season. So if you made any IC post (including flashbacks) then you'd need to pay for the current seasonal expenses.
The
flashbacks lore on the other hand says this at the end:
Important Note: If you are at all active during the current RP season and posting then you are required to pay seasonal expenses for the current game season. If you are only playing in flashback threads, you do not have to pay current seasonal expenses. Flashbacks do not require seasonal expenses because someone cannot collect coin in a flashback thread.
So it looks like the flashback page needs changing to say that you do have to pay seasonal expenses.