It doesn't say that range isn't improved below 1000bps, it just doesn't show any more test results below 1000bps.
And besides that it doesn't actually show that there are no difference between 1000bps and 2000bps either. It just lists them both as being >150m which doesn't necessarily mean that they are equal.
The tests made by the author applies to the modules he tested with and it doesn't necessarily mean that all cheap eBay modules have the same characteristics.
For what I know, if the modules can keep up with the baudrate, it has no use to make the baudrate lower.
Suppose the receiver and transmitter have good antennas and both can do very high frequencies. Why would the range be different with a higher baudrate ? Only when the 433MHz gets disturbed by the modulation.
You may be right but why is there a difference in range between 2000bps and 5000bps in that PDF when it works fine at speeds up to 7000bps in the authors tests?
I don't know if they get more sensitive to interference at higher rates or something but the tests show that the range is better at lower speeds.