Showing that production technology exhibits decreasing returns to Scale
They are two different concepts I think. 

Diminishing product means that holding other things fixed, one unit of extra input($K$ or $L$ here) yields less and less additional output, which you have known. 

Meanwhile DRS is saying that if you multiply both $K$ and $L$ by some scalar $ t &gt; 1$, the corresponding output would be less than $t$ times the original output.