" but I was assuming they'd put it up off the floor a little bit. "

Unfortunately, assuming most of the time ends up this way.

" I'm worried this thing is just going to be one just rotted-out piece of wood by the end of next year. "

Looks to be cedar so it "should" not rot that quick but it will look like hell from soaking up the water.

My real concern is this free standing? Did they anchor it to anything and how? You do not want it becoming a projectile. Hopefully it is not attached thru the roof.

It's anchored to the side wall, not the roof. But the more I look at the more I'm thinking it's more trouble that it's worth. The slabs are individual pieces which means water is just going to get inside the unit, which is hollow anyway. No storage or anything.

"You may need a TV cabinet like this, which is movable and has intelligent lifting function."

I do not see where the OP said anything about using a TV there or had a question about one?

Thank you @TV lift cabinet by intellicube for that sales pitch but I'm all set

Wish you all the best. The terrace bar is really great.

Looks like it has room to be raised enough to get a non-porous material under.