Without qualification, I'd assume they were quoting historical prices. It would be pretty confusing to present any kind of adjusted figures without clarifying them.
It looks as if the Concorde was roughly $5K each way (or $10K RT) +/- in today's dollars--although I'm not sure a standard CPI inflation calculator is the best way to look at airline ticket price changes.
Given the existence of business class only BA NY to City of London flights, there probably is a market for expensive supersonic routes, but it's probably still pretty small. International lawyers may value the back and forth in a day, but most people who fly business/first are probably fine with comfortable seating and a nice meal even if it takes a bit longer.
That's about $16k when accounting for inflation, so $20k isn't too far off.