The linked twitter account is an AI influencer, so take whatever is written with a grain of salt. Their goal is to get clicks and views by saying controversial things. This topic has come up before, and my hypothesis is still that GPT-4 hasn't gotten worse, it's just that the magic has worn off as we've used this tech. Studies to evaluate it have gotten better and cleaned up mistakes in the past.
As to why, it's probably a combination of trying to optimize GPT4 for scale and trying to "improve" it by pre/post processing everything for safety and/or factual accuracy.
Regardless of the debate as to whether it is "worse" or "better," the result is the same. The model is guaranteed to have inconsistent performance and capability over time. Because of that alone, I contend it's impossible to develop anything reliable on top of such a mess.