Well, I read the letter, and I could not say it “orders” them to appear. The letter uses the word, “request”, although the effect may be similar.
Although the letter “requests” their personal appearance before the committee, it appears to demand all the backup accounting data and analysis. If I were the CEO’s here, I would delay until the last minute and then accept the invitation to appear. I would have the requested documentation delivered the morning of their scheduled testimony. Then show up with a Power Point presentation that catches the Dems totally unprepared.