That ain't true.
Backfeeding through the main panel with a suicide cord is illegal in some areas (should be in all areas), and backfeeding without disconnecting from the utility drop is also generally illegal.
But a backfeed with a proper cord or wiring, plus a break- before-make connection should be perfectly legit. And perfectly safe. Farmers have been using huge double-pole/double-throw switches for eons when they backfeed in an emergency.
Gideon
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Noon-Air wrote Backfeeding through the main panel without a transfer switch is dangerous as hell and in most places illegal.