Even if that's the highest achievable, IQ tests designed to be usable for everyone tend to start getting really unreliable above 135 or so. Above that level, you're getting into the territory where test takers basically answer almost all of the reasonable questions right and where those few wrong answers necessarily start having much larger effects on the final score. Because of that, other issues start dominating the results. Random variance, sloppiness, mechanical issues in timed tests, how well you slept, whether there's slightly bad/ambiguous questions, whether you've seen that type of question before, whether you're good under pressure, cultural differences, etc.

For example, one serious official test I did included a timed test where you had to decode a string of symbols to text based on some key. I ended up being fast enough at decoding that I was effectively limited by how fast I could scribble down the decoded texts and turn the page back to the code, not by the mental calculations. I imagine others would have had similar issues, making this particular test at best capable of indicating a wide possible range of mental ability, a range which should be particularly inaccurate around the higher ranges as the ratio mentalTime/mechanicalTime decreases.

There also just aren't enough high IQ people to really effectively study the accuracy of these tests above those levels. There are no good baselines to compare these people and no real ways of checking whether the differences actually matter in any meaningful way.