Does more variable input cause marginal product to decline?
The two statements are similar.

For the website example, you could say that the marginal product of the first 5 workers is 50 units. For the next 5 workers, however, you could deduce that the marginal product is less than 50 units (otherwise "the next 50 units" would require exactly 5 workers, not 10 workers).

The website is merely stating that, in order to get the same number of outputs (in this case 50 units) you need to hire proportionally more inputs. To judge whether this is counter productive requires knowledge about the price you could sell these products, etc.