In flutter/engine/src/flutter/lib/ui/geometry.dart

  /// Whether this size encloses a non-zero area.
  ///
  /// Negative areas are considered empty.
  bool get isEmpty => width <= 0.0 || height <= 0.0;

When width is <= 0.0, then the result is true. But a size that has a width that is equal to or less than zero is not a "non-zero" area; I think it is a "zero area".
When height is <= 0.0, then the result is true. But a size that has a height that is equal to or less than zero is not a "non-zero" area; I think it is a "zero area".

  /// An empty size, one with a zero width and a zero height.
  static const Size zero = Size(0.0, 0.0);
  print(zero.isEmpty)  // true

So, I think that the correct description should be: "Whether this size encloses a zero area.".