If net profit was 0.5B, and next quarter they lost regulatory credits, it would make even more sense. Tesla would have a negative net profit next quarter, all else equal.

I think the author just wants to say that if the regulatory credits are not there, then the profits will be 0.9B less than the reported net profit today, which is about 0.5B. This is a fairly reasonable statement, and it is assuming that they sell the same number of cars and people are already paying the maximum amount they are willing the pay.

You could argument that if the credits are not there, Tesla will sell less cars, and the net profit will be even more slim.