It's assumed that deck posts are being affixed to a wood base, usually the support structure beneath the deck, not concrete. I can't see why you can't use a fence post instead, although, given there's a drop, I'd probably go for 100mm fence post with a 100mm bolt down wedge grip holder, which you can get, just to make it stronger, but that's up to you. This only works if you don't want a slotted version of the deck post to insert supports for the balustrade string though.