That's honestly not particularly clear. It doesn't say the command will be invoked by a shell on the remote host. Sure the whole "separated by spaces" thing sorta implies it will, as spaces don't mean much to anything but a shell, but it's still fairly vague.

In fact, later on the man page only mentions a shell in the part that talks about the behavior when no additional arguments are given:

  When the user's identity has been accepted by the server, the server either executes the given command in a non-interactive session or, if no command has been specified, logs into the machine and gives the user a normal shell as an interactive session.

The wording "executes the given command" would generally not imply "I'll just throw it at $SHELL and see what happens".

A few lines later it gets even more confusing:

  The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed.

...which I definitely would say suggests that either a shell is executed or the command supplied as argument to ssh. That it means "command as interpreted by a shell on the remote host" is far from obvious.