If Haskell is just a signal that selects for more competent people, companies will be happy to hire people whose competence they would have to invest a lot to match otherwise, and developers will be happy to get a larger salary than they would be able to get otherwise. (But yes, this would disfavor people with other kinds of signal.)
If it selects for high productivity people, or if it helps people be more productive, companies will be happy from getting more output than they would otherwise, and developers will still get more money.
The stereotype is that Haskell jobs pay less, and developers are happy to work with a better language. That still doesn't disfavor any party, but I don't think anybody has any evidence that this is the case. Notice that the GP is talking about people competence, not salary.
If Haskell is just a signal that selects for more competent people, companies will be happy to hire people whose competence they would have to invest a lot to match otherwise, and developers will be happy to get a larger salary than they would be able to get otherwise. (But yes, this would disfavor people with other kinds of signal.)
If it selects for high productivity people, or if it helps people be more productive, companies will be happy from getting more output than they would otherwise, and developers will still get more money.
The stereotype is that Haskell jobs pay less, and developers are happy to work with a better language. That still doesn't disfavor any party, but I don't think anybody has any evidence that this is the case. Notice that the GP is talking about people competence, not salary.