I covered that in my post, but I can elaborate.
The existing problem is that rural Americans have more power than urban ones, due to smaller population states tending to be rural and getting more representation per capita. Bringing in a small state that is entirely urban would help balance that back out.
That is the main political divide in our country. That’s how the parties largely divide: rural Republicans vs. urban Democrats. The system is currently biased towards the rural Republicans. Nate Silver once quantified that bias as around 2-3%, meaning that’s how much of a handicap Democrats have to cover.
People who complain about DC statehood being “unfair” nearly always cite that they would (most likely) bring in two voting Democratic senators and one voting Democratic congressperson. The idea is that this gives more power to Democrats–i.e. the party favored by urban America. But, if the system is already biased towards Republicans, the party favored by rural America, that argument becomes less tenable.
Other than an argument about representational voting power, I don’t see any other reason you can argue that making DC a state would be unfair. It’s not even like bringing in DC would overcome the gap completely.