If "drivers of private vehicles in Japan", must understand these "four symbols" then these are not "Japanese symbols that speak without words".
Not one of the symbols can possibly be understood as to its intended meaning without learning what the symbol represents - that is to say, simply by looking at any of them in no way whatsoever suggests, hints, or shows their meaning in the appropriate context.
I don't think they meant that the symbols should be universally understood without need for explanation. That would be accomplished separately through some sort of public education campaign. In the case of the "help mark", they actually explain what it means in multiple languages in a big sign right above the priority seating [1].
All language has to be learned