When he turned his main breaker off it isolated the utility,
then lets think about what would happen if he didn't turn off his main.
If you try to backfeed a utility with a small generator whats actually going to happen.
lets see 220 volt and a 30 amp breaker on the generator; and lets assume a 13,500 volt primary feed,
13,500 volt and 0.5 Amps, of course the inrush to load the transformer blew his circuit breaker or stalled his generator.
So he could not backfeed thru the transformer, with enough generator it would be theoretically possible if there is not a short
in the system.
I will admit to "backfeeding" my main panel also, I verify that no voltage is present on the main,
Then I turn the main off, plug in my cord to the 50 amp receptacles,
bring the generator to speed and close the breaker on the generator then the breaker to the receptacle.
The worst point is that I have a 600 Volt 200 Amp 3 pole double throw switch just waiting to be installed,
which may never happen, I am going to install one of these as I don't have to pull the meter to install it.
Manual Transfer Switch Kits | Generator Interlock Kit
So to all of the "safety gestopos" on here, stuff it.
verify the voltage and check for voltage on the incoming main