This question was migrated to Arduino.SE but apart from that the Atmega328P happens to be on an Arduino board it has nothing else to do with Arduino regarding the problem stated in the question.
The question itself is about USART usage on register level no Arduino library is involved.
A good test whether a question is a arduino user-level question is whether you can replace all mention of "arduino" with "microcontroller" or "microcontroller development board".
If the question still makes sense, then it's not about the arduinoness. Of course then the OP should have done the substitution in the first place to not give the impression of a arduino-specific question. When "arduino" is mentioned, particularly in the title, then you can't really blame people for closing or migrating without looking at it more closely.
On the other hand, if the question makes no sense without specifically mentioning "arduino", then it's about the arduinoness and doesn't belong here.
The question you linked to fits into the second category. Especially considering code was shown, it doesn't make sense outside the arduino context. It was correctly migrated.
void loop() { // put your main code here, to run repeatedly: is straight out of Arduinoland.
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