Everything refers to existence, and nothing refers to non-existence. They are not the same.
They are the same. Because something only exists relatively to what it is not. (Without left, there is no right)
And therefore existence itself is relative, relative to non-existence.
Existence cannot "exist" without non-existence.
The Absolute is neither "existence" nor "non-existence".
In some sense yes, Nothing and Everything are different, ok...
..Nothing and Everything are as much different as "minus infinity" and "plus infinity". Because basically the problem is, we cannot talk about "pure infinity".
But if you make the mental effort to see it, you will see that minus infinity and plus infinity both points to the same infinity.
(if you have difficulties to see it on a straight line, just imagine your sraight line is actually a circle with an infinite radius)
Everything and Nothing are the same thing/ not-thing, seen from a different point of view.