The 200 LC just hasn't been around long enough for many people to get 300k miles. Since 2008 you would have to drive 20k miles per year to hit 300k by now. Sure, some people do that, but it's way over the average mileage for drivers in the US (and you would have paid about as much for gas at this point than the Cruiser cost originally!). So of course mechanics aren't going to see many 200s over 300k. Mechanics aren't statisticians, their opinions are purely anecdotal and fraught with bias (mechanics by definition work on broken cars).
There's also simply unrelated reasons that cars leave the road: crashes, theft, disaster, rust. This will chip away at the number of Cruisers on the road from day 1, through no fault of Toyota.
As for the iseecars, it's inherently flawed as far as I can tell. Most of their "Cars Most Likely to Last 250,000+ Miles" list is just car models that have existed for a long time. The Land Cruiser has been around 70 years, of course it's going to have an advantage getting to 250k vs models that have existed less than 10 or 20 years. Especially considering the 100 series was the best selling by far (so a disproportionate amount of the total Land Cruisers ever made) and they are all around the age that any left on the road will be around 250k miles if they drove the US average number of miles per year.