Nietzsche was very serious about what he wrote in that maxim, you cited, and he truly meant it. There is not a hint of sarcasm or irony in it.
Which tree is strongest? That which has bent but not broken or that which has never bent nor broken?
"That which does not destroy me, makes me stronger". That which has learned to bend, is stronger than that which has not.
Nietzsche was all about pain, torture and misery, because that is what would make the right people stronger, and thus weakness could be overcomed, and people could be able to bear other nations, and people in general, in their hearts, and people would then rather die than live in a state of hate and fear, and thus common rationality could be brought back as the domineering instinct in peoples, instead of rabid pain giving expression to mutual self-destruction.
Thus, he very much meant that people grow stronger by the things which makes them bend without destroying (killing) them, like the sturdy plants which has been exposed to hard but fitting wind.
Nietzsche was certainly not a pacifist by any means, but only one who saw that true happiness and peace can only come by by the means of an overfullness and strength to lay down arms, to laugh at murderer of you, if you cannot conquer him, out of super-rationality, embodied in Supermen.
"Haha, you cannot kill me, because I live on - even beyond you! Now I would rather die than live in a state of meaningless fear and panic - so kill me if you would bear arms, but know that I will try to kill you first. Thus, the victor and therefore the strong has survived, and this is the just thing. Let us be champions or be dead!"
Thus, the strongest must be the one has bend the most, that has come home from war victorous the most, and which have even conquered himself at last. Then, will he lay down arms out of a feeling of value, in that his aggression would only do damage to himself, or in other words his eternal life, and he would rather perish than scar his eternal life - or his eternal joy, eternal feeling of conquest and power. "I will only fight the battles that are meaningful to me or else I will gladly perish - and your fight is meaningless to me - therefore, goodbye!"
It's the same with muscles - that which bends (exercises) but does not destroy (ruin, injure f.ex) muscles make them stronger. The muscle that has never been to war, in some metaphorical sense, and come out alive is weak, whereas that which has done so is strong. It's the same with the mind - train it (in pain and under pressure) and you make it stronger, if you do not destroy it, that is, and this is what Nietzsche meant by his aphorism and the long quote above noted by someone else.
Only he who has been to war and has come out alive is strong; he who has never been to war or has perished from war (in some way and to whatever degree) is weak.