The soft wood/hard wood thing is all about sap in the wood. If you tried to use a soft wood like cedar, there'd be an unbelievable amount of sap in the wood--it's pink. The sap would never come out even if you boiled it--it's a mess. You probably wouldn't want to use Pine, either.
But "hard" woods like, Oak for example. You cut a tree down, it's moist on the inside and there's some kinda sap in it, but it's nothing like cedar. You let Oak dry out for a year, the sap comes out, and it won't leach all kinds of tree sap stuff in your tank. A cedar fence plank 3-4 years old will still smell like cedar if you crack it open. Not so with Oak.
I think if you have an obviously old and weathered piece of driftwood like in your pics, you're OK. to be sure, cut into it 1/8 or 1/4 inch, be sure it's not green or fresh down deeper. If it's dry that deep, you're good.
Nice driftwood by the way. I'd love to just find that while I'm out and about. It'd cost you hundreds if you bought similar pieces at an LFS or the Internet.