It very much makes a difference, because it's a useful tool to quickly and easily gauge the difference in skills in a heated multiplayer match. Nobody is trying to set any records, so the only performance that matters is the relative performance, which is well satisfied with the 1m score cap and its attached mechanics.
Meanwhile leaderboard scores are about playing absolutely perfectly, making misses far more punishing in longer maps, with no need for quick and immediate comparison. Getting a very high score is extremely satisfying to the player as their reward for playing perfectly for a long time. The player wants to feel awesome for doing so well, and a very high score is one of the best ways this is represented. They then feel awesome, because they've finally set the cool record score they wanted, with a score of 30 million points! That's much more satisfying than a lame, uniform, 1 million points.
But, overall, I'd agree. What difference does it make? Why is this being changed if it doesn't make a difference? It makes literally no sense. Other changes? I may disagree with them, but I can see the arguments for them. There's literally no reason to add a score cap to singleplayer.