I got 1200% without trying, without putting much thought into it. You can see all over social medias people winning by accident, from "pro" players that optimize their play, to casuals that barely understand the game mechanics. It doesn't require any kind of effort, it just happens and it's annoying.
I won around turn 190 while actively trying to stop my cultural victory. I would have won a lot earlier than that if I just moved a couple of relics to a city with reliquaries.
You can get a 125% modifier to Civs that you have an active trade route with them. That requires you to reach that Civ with a trade route, which takes a lot of planning ahead, at least to reach far away Civs in larger maps, and usually it isn't possible to reach every Civ (depends on the map). You also need to keep the peace, so you can trade with everyone (same for the 25% from open borders). For most of the duration of a match, it's a 25% bonus. To get the full bonus you need to recruit 2 Great Merchants, one from the modern era and one from the atomic era, and you need to slot an Information era policy. It takes planning, it has a cost, it only comes at full effect in really late game, and it usually won't apply to every Civ. You can get considerably higher modifiers from a monopoly without even knowing monopolies are a thing, way earlier than you'll get this 125% from trade routes. Any other modifiers that are that high apply only to tourism from specific sources, which limits their effect considerably.
The tourism we get without this game mode is already high, mainly considering that the AI can't keep up with their culture. Adding another modifier to the game without reducing the others is, on itself, a bad idea, which makes an easy victory easier. A 25%-50% potential modifier that applies only to Civs who do not control an instance of a resource would be aligned with the expected OPness from the New Frontier Pass, turning it into another unhealthy, but acceptable feature. Anything above that is too much. And no, getting ridiculously high modifiers isn't an edge case. People posting screenshots of these ridiculously high modifiers or complaining that they won a culture victory by accident is top 3 most posted thread on Reddit, right there with "Kublai Khan breaks policies" and "Look at this preserve yields!".
While they nerf it, they could also buff the gold you get from monopolies, which is underwhelming.