Read "not just twice" as "not only twice".

Then you can read "it's twice squared" as (the first use of twice) + ("twice squared")

The 'it' reference can change. The first 'it' is referencing the difficulty. The second 'it' is referencing the first 'twice', creating the option that 'it' is further "twice squared". Not the most elegant interpretation but a possible interpretation.

Learning to avoiding this vague language is why math majors should still take the occasional poetry class.

English is a strange language. "Twice squared" could be interpreted both as "Twice as hard, squared" or "Twice as hard, squared two times."

Think of the sentence, "A man, twice fooled, shall not be fooled again."

If x is the difficulty, the phrase either means “twice the difficulty, squared”: (2x)^2 or 4x^2.

Or “the difficulty squared and squared again”: (x^2)^2 or x^4.

The GP is asking why x == 2 in the second example rather than just “the difficulty”. English interpretation doesn’t explain that.