man echo relates to the echo program. GNU echo supports a --help option, as do some others. When you run echo in Bash you instead get its builtin echo which doesn't.
To access the echo program, rather than the builtin, you can either give a path to it:
/bin/echo --help
or use Bash's enable command to disable the built-in version:
$ enable -n echo
$ echo --help
Bash has built-in versions of a lot of basic commands, because it's a little faster to do that, but you can always bypass them like this when you need to.
Source: https://unix.stackexchange.com/questions/153660/why-echo-help-doesnt-give-me-help-page-of-echo
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