First of all you got to take these things with a pinch of salt. People are trying to run a business and this is a marketing avenue. It helps sell product if your product is designed for the market.
On tires, it depends which tires you are running. But if your championship runs Le Conts, then you are running the same tires as the OK World Championship. Now there will of course be a difference in the amount of rubber going down, just based on the number of people on track. But a lot of regional and national championships in the states have more entries then the CIK championships so its probably not that.
The difference, if there is anyway, is more likely to do with power output and deliver. If we are assuming that the Euro manufacturers only make chassis that work for the class they race (rather then the classes they sell the most chassis in) then the chassis would likely have a very “stuck” rear end because OK engines are just way more powerful then X30 etc.
However, the US is by far the euro manufacturers biggest single market so I would contend that most euro manufacturers are making chassis for a rubbered in track with an x30 or equivalent power plant. And that’s why most karts, when tested brand new on a clean track, have a very loose rear. You could argue that LO206 is not a market they might be bothered to design for (as it doesn’t exist most anywhere else), but in some cases they have.
The IAME SSE comment, just sounds like a sales pitch to me.
The big disclaimer with watching Euro racing though is that you don’t know 100% if the chassis winning those races is the same as the chassis you will buy. The manufacturers can changes the tubing as much as they want for the races they are doing and often do. A careful eye will show that some chassis didn’t even have time to get painted before being packed to go the race and that’s because they are trying something. Then even when they painted they might be different. When I race Formula A, I turned up at one race and one of the chassis to test was a CRG road rebel copy.