It seems that this statement can lead to different conclusions based on one's biases for or against Musk / freedom of speech:
1) The code base is a mess because the smaller, current team don't know what they're doing and/or management (i.e. Musk) is pushing them too hard and making poor decisions
2) The code base is a mess because the far greater number of engineers there before didn't know what they were doing and/or management weren't pushing them hard enough and were making poor decisions.
Both could be true but I'm going with something the more complex view that it's a little bit of everything, and that Twitter does seem to be moving in a positive direction overall.