Question

If rectangles must have right angles explain how a rhombus could also be called rectangle?

Answer 1

A rhombus MAY be a rectangle.

Explanation:

The rectangle (right angles) is really just a special form of the rhombus - not the other way around.
Rhombus Definition:
A 4-sided flat shape with straight sides where all sides have equal length.
Also opposite sides are parallel and opposite angles are equal.

https://www.mathsisfun.com/definitions/rhombus.html
(and interactive graphics)