It does change the output on my setup:
x = torch.rand(5,5).cuda()
print(torch.topk(x, 5, sorted=False).indices == torch.topk(x, 5, sorted=True).indices)
> tensor([[False, False, False, False,  True],
          [ True, False, False, False,  True],
          [ True, False, False,  True,  True],
          [False, False, False, False,  True],
          [False, False, False,  True,  True]], device='cuda:0')