This is very wrong. Games are about game structures and procedures--math only matters in service to these procedures.
Which is a bigger change to Monopoly: doubling the amount of money you get when you pass Go (change to math), or letting you choose whether to roll one die or two on your turn (change to procedure)?
D&D has several core mechanics and procedures such as gaining XP and leveling up; dying when you run out of HP (and fail with death saves or whatever last-chance mechanic the DM is using); entering a room in a dungeon/scene to trigger actions from the things in the scene (some DMs will have offscreen monsters actively doing things but it's challenging--it's an almost universal convention to wait until players can observe things, except for stuff like loud noises triggering monsters to wander closer).
I haven't paid close attention to OneD&D but I did notice WotC fiddling around with stuff like the critical hits, in a way that has a hugely deleterious effect on the Champions in particular and a significant impact on the DM (no crits for monsters) and all players (death saves become safer if monsters cannot crit). These are large changes. I think some of them have been rolled back but like I said I haven't been paying close attention.
Games are far more than just "the math" though. Even sometime as simple as changing who rolls saving throws (GM or player) can have a large impact on the experience of play.