Hi @d0rkpress, welcome to WordPress Trac! Thanks for the report.
Just noting this has been previously reported a few times, most recently in #40667.
As stated in the handbook article you've linked to, we don't consider usernames (and by extension, the existence of accounts) to be private. A similar thing can be achieved just by browsing the /author/{slug} views.
We need to balance user friendliness with information disclosure and as usernames are not considered private information, user friendliness wins here.
Please don't ignore the warning that Trac displays when creating security tickets. If you believe you've found a vulnerability, please disclose it to us privately, via HackerOne.
Related: #3708, #4290, #5301, #12129, #22421, #27125, #31787, #40667.