Contrary to many of the comments here, I think they did a good job of considering what it takes to make something foreboding without sounding like "here be treasure." The pseudo-mystical messages sound hokey, but they're effectively a backup system in case the straight-up "This is atomic waste, here's a description of atomic waste" descriptions are incomprehensible to future generations. And the more primitive communications deserve more consideration, because that's the harder part.

Additionally, I don't think the "no marker, anonymous patch of ground" plan is sound. 10000 years is a long time, which will hopefully be inhabited by peoples more advanced than us, and they could do a lot of digging in that time.

That said, the approach I'd suggest would just be a big plain monument that's physically obnoxious to get around. Although the insides of the pyramids have been robbed, the pyramids themselves will last another 10000 years, and I doubt anyone will try to mine under them during that time. And experience has shown that the best way to preserve a language is to make sure there's a large enough sample for someone to brute-force it, so these pyramids could contain chambers full of detailed explanations with pictures.