It doesn't though. It was going off usage of very common words like "its", "he", "and" rather than topic specific ones. Just that alone seems to work shockingly well. If you combined it with a few more data points like timestamps and topics of interest it would get even more accurate.
I can guarantee you it doesn't work in practice. If you put aside my former account, it mostly matches the current one with other rust developers and absolutely not with my alt (which doesn't discuss Rust at all).
I'm not questioning what would theoretically be possible to do, but the one that I saw failed the test.