If you're taking advantage of the 120% rule, then the backfeed breaker needs to be as far away from where the main grid breaker connects to the bus bars as possible. The statement put it at the bottom assumes the main breaker feeds into the bus bars at the top of the panel. Which is often true in most modern panels, but isn't always the case, especially in old panels. Then there are panels where the grid feed breaker goes in a slot like any other breaker and the convention is to put it in the top left slot. But nothing forces that to be the case. The trickiest case is where the grid breaker is in the very middle of the bus bars. Which is common on some older combined meter box / breaker panels.
If you're not taking advantage of the 120% rule, then you're free to put PV backfeed breaker anywhere you want.
The reason you put it as far from the grid breaker as possible with the 120% rule, is because statistically you're very unlikely to overload the bus bars with that arrangement. Because all the breakers between the two power in-feed points are nearly guaranteed to pull off at least 20% of the total power. So at no point on the bus bar are you likely to exceed the bus bar power rating even if both in-feed breakers are running at 100% of the breaker ratings.
