I've said it many times before, others have come close to saying it here, but...
The answer to the thread-starting question is: the SOS is unambiguous.
The strobe MEANS nothing. Yeah, it might sort-of mean "I am here" on a dark highway, but, off in the distance, there's no reason not to ignore it, no compelling reason to respond. It's a freaking flashing light in a universe full of flashing lights. No inherent meaning.
The SOS is entirely different. It's a distress signal. It means that you are in dire need of help. It can have no other meaning unless it's misused.
Put yourself in the place of somone seeing each in the distance, on the water or in the mountains. Are you going to go miles out of your way to respond to just a flashing light, compared to a universally-recognized distress signal? Probably not. Are you going to call the Coast Guard, Rangers, police, whoever and report that you saw a flashing light? Get real. Report that you saw an SOS, and you'll be taken seriously.
Why "attacked"? If the only danger you can think of is someone attacking, I guess you've never been too cold, too hot, too tired, thirsty, hungry, sick, injured or in pain. Yeah, SOS may not be much use for the quick ones, but there are many, many slow ways to die.
Don't dismiss the implications just because you were born in the most protected cultures in the world, in the most protected generations that have ever existed on Earth, and you may not personally have ever been in the military, ever gone off the beaten path, been a Boy Scout or even really been camping or backpacking. Think it through for yourself. SOS works, and has for generations because it means something very specific.