> But it turns out NHTSA disputes even Tesla's more mild assertions:
I think it's worth noting that neither are contradicting each other, as they are saying different things. Tesla says NHTSA testing shows something, which may have nothing to do with specifically what the NHTSA says or how it rates. If they have access to the actual test data (or the tests are publicly available), they can make assertions off that data.
If that was the case, the NHTSA is clarifying its position, in that it is not making those claims so them saying "NHTSA does not distinguish safety performance beyond that rating, thus there is no 'safest' vehicle among those vehicles achieving five-star ratings." is a way of clarifying their position and staying neutral, since they haven't made that assertion for any other vehicles in the past.
In other words, it's possible for them both to be be correct and accurate, but talking about slightly different things. This doesn't even require intent to deceive from either party.
I, of course, have no idea if that's true and this is the case. It is one possible way to interpret the statements though, so a little additional investigation might be warranted before we determine that either statement is false or intentionally misleading.