Sloping line refers to any straight line that isn't vertical or horizontal, hence the top rhombus has two horizontal and two sloping lines all required to be the same length. The length of the sloping line in a candidate rhombus can be calculated using Pythagoras' theorem. A compass could alternatively be used to find sloping lines of the same length as a horizontal line.
To complicate this further, I was taught 50 odd years ago that a rectangle with equal sides is a square rectangle, which now seems to be just called a square.
Let alone all the other rules. Apart from not meeting the required '20' cells area, and making a rhombus with the letters MISS in the corners, I convinced myself I got it right.
Only just got around to looking at the solution on the Times website. There is another rhombus which will do. The vertices are at the centres of these four cells: L of ILIAN, S of ESTATES, S of DIECAST, R of DEADCART. This has a length of 10 and a height of 4. It entirely encloses the rectangle but does not touch or overlap it. I assume that the word 'overlap' in this context means 'intersect'. As the preamble says: All items are formed of straight lines. As such, the figures are perimeters not laminae, in the same way that a circle is the circumference of the figure, not its inside (for which the correct name is disc rather than circle).