Who was this written by, a Brit?

Nope. Likely an American.

When cooking, people in general like to use round numbers, like "200°C", since a difference of 5°C in oven temperature is not a big deal.

And yet they went with some oddly specific 205°C. That only makes sense if they're used to Fahrenheit, eyeballed a round value (like 400°F), converted it into Celsius (204.4°C), and then rounded it up to discard the decimal.

I'm also going to say they're completely clueless when it comes to cooking - 200°C is the oven temperature. The chicken itself reaches a far lower temperature, in the 70~80°C range. By the time the chicken reached 200°C, it's already dry and close to catching fire. (The self-ignition temperature for biological stuff is typically between 200°C and 250°C.)

Are you sure? The numbers in the ~~tweet~~ reddit post talk about total mass and heat capacity. So I think that means the entire bulk has that average temperature.