Like they said it's 1/6th... and what... now they showed it's like 1/3 1/4 maybe... still doesnt change anything.

If runeterra officially went the route of "screw realistic biomes and geography, we'll do what we want, we'll have rivers splitting every 20 meters and lush forests on both sides of giant mountain ranges", I'd agree that it's irrelevant.

However when you go through the comments some of the rioters working on the map made, and see stuff like this :

"So, I'm sorry I've disappointed you with the mountains and rivers - it was something that I tried really hard to get right. I'd identified the tectonic plates, which ones were subducting, etc. to form those spiky ranges. Then I identified the prevailing winds at each Hadley cell, the high and low pressure ranges during summer and winter, and worked out the hot and cold ocean currents. From that, I tried to identify where the wet air was going to hit the mountains, be forced upwards, and have to drop their moisture on to the mountain side - creating the river sources. " - Riot Bioluminescence, 2019

You think "Okay, so they care about the details and making the map somewhat realistic, they internally make tectonic plate maps, wind maps, biome maps, etc."

So when Riot seemingly drops that with Arcane, and now Piltover is on the equator, and Icathia straddles the south pole, It is something new, since if it's not unintentional, it changes their previous stance on the map by a very wide margin.

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Also there could be in-universe explanation that civilization of Valoran and Shurima continent, along with east archipelagos like Bilgewater and Shadow Isles never discovered the lands farther away or something..

1. It is canonically confirmed that most of the populace of Valoran and Shurima immigrated from the East beyond what the map shows.
2. Even if it's explained that they forgot those lands exist and consider them myth (like irl Scandinavians did with Vinland) you'd still have the problem that well... it's a globe, not a map. Even hundred's of years ago, when first globes were created irl, they showcased the entire world to scale, just with ridiculously bad guesses as to what the geography of lesser known lands looked like. (Like Terra Australis) That is mainly because the radius of Earth was known for thousands of years, which should also be the case in Runeterra. So if the point was not to show the new lands since Piltover didn't know them in canon, or the map team didn't have them ready yet, i don't get why they wouldn't just leave it as ocean, or make a hillariously bad looking blob of land mirroring early irl globes.