But then what does it mean when the FT writes that that "revenue" number already accounts for the 15 billion losses? That the FT doesn't use these terms correctly either, and we can't really know what its reporting means?

It’s trading profit. No one is counting their gains from a trade while including their rent into it.

It’s a much easier to understand metric when you are discussing losses on a trade. No one is calculating losses including salaries.

It means Jane Street is still highly profitable in trades this year despite making a bad bet. If they have overhead that eclipses that, that is an entirely different metric.