RNG is semi-manipulable in the mode. It's not fully able to be manipulated (so you can't force spawns), but you can definitely influence it. Your levels and what champions are bought influence the pool, so you can buy some champions to reduce the odds your opponents get them and to raise the odds you get champions you want. If you do it right, you might even be able to influence what your opponents build, if they build based off of early game.
There's also positioning (how you position can very well make or break a fight) and there's quite a lot of nuance as to how you run the match overall (Econ versus rushing levels versus hyperrolling, what traits you run and how you run them, whether to win or lose early...)
Yeah, in low elo a lot of that strategy is able to be ignored fairly safely, but as you go up it starts mattering a lot. RNG plays a heavy role in it, but, much like in normal League, luck can only win or lose individual games, and it's your skill that determines your winrate.