No, this is a design problem. The fact that so many missed it is just evidence of how subtly tricky good design is.
By the time you actual visit the site you ALREADY have an idea of what it's supposed to be about, the colors draw your eyes to "Here's how you can tell your score, press here to start" and skip the fine print entirely. People MIGHT skim the "fine print" that is the actual instructions with key details, and what do you read when you skim? The first part of a paragraph/sentence to establish a subject and action. And what's the First part of that sentence? The same thing the title of this post told you.
"Oh, yeah, I already know this"
Skip to the pretty colors telling you how to score your guess and start the game.
EDIT: Also, the UI after you click through to start also assumes the user read the instructions, instead of trying to organically guide them to the same conclusions. /EDIT.
Cue so many people wondering how they could miss something laid out so clearly in front of them.
That is, the more stuff is usable without reading manuals, the more people ignore the manuals - making it harder to build complex, powerful tools, and in fact creating a strong design pressure to "fisher price" your product. Overall, this trend ends up disempowering users.
If folk aren't seeing the text laid out in front of them, there are a couple tricks - demonstrate, make the instructions pictographic/difficult to dismiss.
There is a fine line between making your game accessible and providing too much instruction, but I can think of a couple visual cues that probably could have been given.
That being said I had no problem reading the intro, on the other hand I will regularly finish those long, long, long 45 minute articles that have "congratulations you made it to the bottom", and I know generally people who are great at reading much, much faster than me have little problem figuring out "techy" things, so I do think in general people just don't read enough.