Actually, the "HTTPS-only" isn't all that useful at present (and I suspect Chromium will soon remove it), since Chromium (and Vivaldi) is now using HTTPS-first by default. That is, if there is a valid HTTPS port for a host, a HTTP URL will be automatically redirected to that URL (think of it as a sort of soft-HSTS). (A difference from Chromium is that entered HTTP URLs are treated the same way).
One can probably consider HTTPS-first as an indirect protection against (at least some of) the HSTS misuse mentioned.