First of all, I think the guy in the article is confusing Greenland with Iceland. Absolutely nothing is concentrated in Greenland.

Second of all, data centers existed before Bitcoin mining, so there's no need to reinvent the wheel. The only viable solution for reusing the heat is to run the mining coolant in several closed loops inside the data center, run them through several heat exchanger where the heat is led to a different closed loop system used for central heating for nearby houses or office buildings. 40-60 degrees celsius through radiators makes sense. Heating the liquid further cancels any gains because it will need to be cooled down again (remember, you can't use the liquid from the closed loop system directly), and water heaters are (as lambie pointed out) used too sporadically.

Third, miners don't do that because sunk cost is a killer in Bitcoin mining. And such a system is very, very....VERY expensive.