“[Whether the ancient Egyptians actually sailed or did not] to South America remains uncertain, but Heyerdahl’s Ra II expedition demonstrated that they could have done so.”

I had incorrectly answered [The actuality of the sailing by the ancient Egyptians]. Is this incorrect because it’s too wordy? Or did they use “actuality” incorrectly?

The correct answer was [That the ancient Egyptians actually sailed]. Can you start a sentence with “that”?

Your answer is wrong for a complicated reason: the “done so” later on in the sentence needs to refer back to the verb “sailed.” When you change that out for the gerund “sailing,” the “done so” refers to nothing.

Yes, you can start a sentence with “that.” If it’s done in a right answer from a real SAT, you can trust that it is actually a thing that can be done. 🙂