It's a bit misleading to say that there was absolutely nothing unusual about the game. The Sinquefield game in question showed a very high correlation between engine optimal moves and the moves played by Niemann. His gameplay accuracy here is within the bounds of what the very top players can achieve in individual games, but high enough to raise some suspicion.

The next step is to place that mild suspicion in the context of both his history of admitted cheating, his unwillingness/inability to explain his remarkable moves post-game, and the additional context of many other games played in the last few years with _extraordinary_ accuracy. Now something that could be explained by just a very strong game appears very suspicious.