In sci-fi currency backed by energy, or in the form of energy itself, is a pretty popular trope, so maybe the last quote was influenced by it. But I wouldn't say that Bitcoin is backed by energy or hash power, because PoW is "wasted" in the process, you can't get it back like you could have exchanged banknote for gold. So, Bitcoin and other coins are secured by PoW, but not backed. There are projects like Storj and Gridcoin that actually try to make coins backed by storage space and computing power, but so far they were not as successful as PoW coins, and there's a lot of criticism towards their security, which I think is valid.
The current state of your coinbases are increasingly backed by the "wasted" energy as more blocks get pilled above the previous one, so personally I think one could use the word "backed".
cellard is indeed correct.
the cost of mining is not locked to the block it mined but also to all previous blocks
after all if someone wanted to counterfiet satoshis first tx to hal finney to make it pay someone else. they cant just use the same CPU hashing speed that satoshi had to create that block.. they would need to re-hash the whole network. and do so with more speed than now just for a chance of catching up
yep they would need to rehash over 529,000 blocks just to change a tx in the first hundred blocks