HSTS is suggested but no warning about making site inaccessible?

[slrslr]

On this page:
https://infosec.mozilla.org/guidelines/web_security#http-strict-transport-security
(its source: https://github.com/mozilla/infosec.mozilla.org/blob/master/docs/guidelines/web_security.md)

I am not able to easily spot a warning which says that enabling the HSTS header may cause the site inaccessibility in the event of a SSL being broken?

Details:
https://internetlifeforum.com/showthread.php?28243-Downside-of-a-HTTP-Strict-Transport-Security-(HSTS)-inaccessible-site&p=60353#post60353

or this is misunderstood by me?