In “Ozymandias,” what makes line 11, “‘Look on my works, ye Mighty, and despair!’” ironic?

The words were carved in a language that no one can read today.
Ozymandias had no idea his words would be quoted thousands of years later.
There is no such thing as the “Mighty” to worship him anymore.
It was meant as a boast, but now it appears to be an admission of futility.