I don't know what the gearing looks like on this car, and I honestly don't care enough to look it up, but why do you and the article seem to think a car with 1750 (claimed) horsepower might be slower than one with 1160 (claimed) horsepower?
It seems unlikely that people capable of building a car like this would have released a claimed speed that their car was incapable of reaching due to gearing. Then again, I've seen a lot of stupid things in my time racing.
1. some engines will heat soak and lose more power than others under extended load, which doing 300mph will take, and the tuatara is a smaller engine running a lot of boost, more likely to lose power over time compared to some of the other hypercars with larger engines or partial electric power boosts.
2. gearing
3. aerodynamics. not just drag but will the car become airborne/impossible to control, etc