Can someone explain the approach they use for this question?

You cannot calculate using 1, because we don’t know whether the two equal sides are sqrt(2) times the third one, or 1/sqrt(2) times the third one

We again don’t know whether the sum of isosceles angles is 135, or the if it the sum of unequal angles

If angles E and D sum to 90, then angle F is a right angle. In a right angled triangle, the hypotenuse can never be greater than the legs, thus the two equal sides are the legs of the triangle. Area of the triangle would be 1/2 x 3 x 3 = 4.5