That is a terrible explanation to be honest. 10pm PT is 6am UTC, which is not a great time for Europeans. Both COTA and Miami started around noon PT, which is 8pm UTC and a much more reasonable time in Europe if you ask me.
Unless they are trying to appeal to Australian fan, my money on the real reason is that they wanted the glamour of a night race in Vegas.
As a F1 fan at the West coast, 6am or 7am are my favorite time for race events. It allows me to watch the race with a nice cup of coffee and still have a full, productive day ahead. Of most races, like in Europe, were in the middle of the day I'd stop watching again like I did when I lived in Germany. I have other stuff to do than depend 20+ weekends a year watching F1
I feel the exact same way. I miss F1 races showing at 730AM EST, it was very easy to get up and have a full day. I barely bother watching the 1PM races.
Unless they are trying to appeal to Australian fan, my money on the real reason is that they wanted the glamour of a night race in Vegas.