The technical feasibility of deep sea mining is not in question. Offshore industry has been able to extend drills, scoops, and other processing equipment to the sea floor since the 1970s. Even the former Glomar Explorer is still in use. Technical capability to drill an ocean floor deposit is not an indication of whether such an effort is economically feasible. One big difference between deep sea oil drilling and metal drilling is the ability of colocated natural gas deposits to force oil through a bore hole to the surface in a confined pipe. Any offshore driller looking to turn those rare earth drill sites into profitable mines will have to consider the energy costs of sucking tons of silt from the ocean floor through at least 11,000 feet of water. Scooping it up is certainly an alternative, but compare lifting a scoop through miles of water versus traversing a much smaller distance on land. Miners can build conveyor systems to carry ore out of land-based mines; building a conveyor system to go vertical from the sea bed would be a huge undertaking with a host of unknowns (like stabilizing it against ocean currents). Engineers may be up to the challenge right up until a driller's finance department figures out how much it will all cost.
Consider the environmental implications of the Japanese research project's findings. If the silt is processed on a surface ship, where will the processor dispose of the slag? Simply dumping it overboard is not a viable option, especially if it's been acid-leached (as the Japanese researchers claim to be the ideal technique). Whichever country ends up granting drilling permits will undoubtedly want to enforce its environmental laws on the seabed. The EPA will probably require slag to be brought down to the seabed with controlled action. Operators should thus plan on doubling their estimates for an energy budget.
Add it all up and filtering through deep sea silt looks like a lot more trouble and expense than prospecting for rare earths on land. It's not impossible, but it needs to be profitable.