"secret" not in the sense that it's hidden, but that most people won't know about it. Because approximately nobody actually reads man pages in their entirety, they just get in to find out how a specific flag works and then get out.

Using "secret" in that sense instantly reminds me of hyped-up headlines for time-waster news stories. Most people don't know what 2^8 is, either.

the difference is that knowing 2^8 is generally not useful to people who don't know it

this here is something that's pretty useful to most ssh users, yet seldom spoken of

a better analogy would be comparing it to calling a very good, but not well-known restaurant a secret place - using the word to mean a hidden gem rather than an intentionally hidden secret