The environment aside China is really making smart moves.
I think you are being oversensitive.
On the surface, it would seem that the engineering requirements for space tend be greater than those involved for the deep sea. But you have a different set of trade offs; for example, you may end up disturbing the natural equilibrium of the biosphere in the deep sea, whereas in space you wouldn't have such an issue.
Also, they would still have to man the station underwater, whereas in asteroid mining, it's relatively easy to send a fleet of drones for resource gathering and discovery. The only station really required would be a fueling station.
It's interesting why you would think this. Underwater drones would be an order of magnitude easier. For starters you can control them with amazing high bandwidth just using a simple 10km long cord from a boat.
You can pull them in for maintenance every day.
They cost ~zero $ to deliver. A fishing boat could do it.
I think because SciFi movies delivers space so much we lose perspective of how hard it is and how 'easy' other things are.
I'm not sure why China is manning this underwater lab other than it's cool. (like going to the moon)
Even at 100 times less cost per KG into orbit it would still be too expensive even for the rarest of elements.
And even if you have a 0$ per kg cost to orbit launch system you still need to find an economical way of getting things down to earth which is even a bigger problem than launching your mining equipment and personnel in the first place.
Underwater, the pressure increases by 1atm for every 10 meters of depth [1]. At the sea floor you're looking at 20 to 400 atm depending on where you go, and that affects both robotic drones and human habitats.
On the flip side, if you're doing underwater drones then they never have to go more than a handful of km away from a surface vessel, so you can stay physically connected to them. That makes power, communication, and retrieval far easier.
Meanwhile HN commentators are sure this is a brilliant move that will catapult them ahead of the world in deep sea exploration.