"Text someone else wrote" is already self-watermarking if it's long enough. The infamous "six lines written by the hand of the most honest of men" may not be enough to hang someone over it, but apparently it's more than enough to uniquely identify them by word choices alone.

Text the whistleblower only reports on, well, if they got it from a computer system, there's already precedent of altering word choices, typos and punctuation in e-mails and memos to create unique per-recipient or per-recipient-group versions, which allows companies to trace leaked transcripts reported by press back to source of the leak.