There is now growing belief that China is under-reporting. Primary evidence for this is that the stats fit too neatly to a curve extrapolated from previous releases. Would actual deaths occur so tidily? Secondary evidence is that China's response seems significantly larger than expected based on the statistics they are releasing. Judge by actions not words, and all that jazz.
This doesn't conclusively prove tampering, but at minimum suggests there is some rate-limiting factor influencing the figures. Shortage of test kits or test lab throughput, perhaps.
Either way, if China is under-reporting it puts the rest of the world at a disadvantage information-wise, and that leads to inadequate preparation and response.
Assuming this is true, Dr. Tedros is in a political pickle. True transmission and mortality rates will become apparent once the virus spreads in countries that can't or won't falsify figures.
On the basis of all of the above, which I admit is by no means conclusively proven, this Tweet makes a lot of sense. It provides a stepping-stone in messaging from fawning over the CCP to providing more aligned-with-reality messaging on the developing pandemic. When the dust settles, I wouldn't be surprised if any stats differences between China and what we see in the rest of the world is pinned on 'mutation'.
A more generous take is that Dr. Tedros had to gush about China to avoid non-cooperation, but is now trying to move past that and back into a messaging space where the WHO can best prepare the rest of the world to respond appropriately.
Disclaimer: I'm not an expert and I spend far too much time in the sewers of the Internet to have an objective opinion. This is just a summary of the more plausible speculation I'm seeing.