It is not a typo, it is in fact what POSIX also says:
> onlcr (-onlcr)
>
> Map (do not map) NL to CR-NL on output. This shall have the effect of setting (not setting) ONLCR in the termios c_oflag field, as defined in XBD General Terminal Interface.
The fact that the mode isn't called "onlcrnl" is probably just to keep the setting names short and consistent (or at least consistently short).
The Rationale section tells us that the standard for `stty` was adopted from System V, so I'm assuming there was backward compatibility to older systems to care about too.