I was the person. I have been writing on the web since 1999. The only website on which writers thank people for reading is Medium.

It has nothing to do with being well-mannered. It's implying that the reader was actually reading your piece as a favor to you. Readers don't read as a favor to the writer. And if the writer actually found your piece stupid and said so, then it's sarcasm.

Feel free to read the article below which tells you more about bullshit writing rules.

https://medium.com/born-to-write/bullshit-writing-rules-4e827d04dcb0

By the way, the length of time one has written on Medium doesn't give one any sort of seniority in the sense that 'one should know better.'