Nonsense is perhaps a bit strong. What people mean when they say Bitcoin is "backed" by energy, is that there is a certain cost associated with producing one BTC. That cost is primary the electricity consumed and electricity has a spot price (barring distribution problems which might skew the price). Miners are not likely to sell under the price of production, at least for shorter time frames, which creates a supply elasticity.

It is not backed by energy in a literal sense of course, that would be obviously meaningless. But the price of energy, BTC and mining difficulty are connected. A change in one variable affects the others.