Is it possible to know the actual computing power (in FLOPS) of the
ETH network or sum of the underlying hardware?
There are two different kind of computational power here. The one spent on Hashing which provides security but nothing more for the user, and the one spent on actually executing transactions.
Regarding hash power, you can only get rough estimates... As long as the network is kept secure, it's enough. I do believe that your estimate is low, the average miner has much more computational power than a GTX 1070... You can check on the hashrates of large mining pools members such as Ethermine.
And anyway, Proof of Work will soon come to an end.
For the useful computational power which I personally find much more interesting, using hashrate is not the way to go. Application Specific Integrated Circuits (ASIC) are contributing heavily to the global hash-rate but are virtually useless for actual code execution, the same goes for graphic cards. What matters is useful computational power for this comparison to make sense with super computers. Having dozens of exaFlops means nothing if we cannot do anything with it.
I think we can agree on that, as you said we're building a peer-to-peer general purpose worldwide computer, plus the comments do use the Fugaku for comparison.
The vast majority of computing power used by Bitcoin or Ethereum is useless outside maintaining security.
So let's try another estimation focusing on useful work that I'm afraid will be very disappointing.
According to this website there are 2462 synced nodes at the time of writing (using this one as in the comments you'd get similar numbers).
Assuming that all of them are full nodes, that would mean 2462 processors currently contributing the useful Ethereum computing power.
Assuming that those processors are 100% devoted to code execution for the Ethereum network.
Assuming that they are all 1 Giga FLOPS such as the AMD 5950x (one of the fastest consumer available processor right now)
That's a combined useful power of... 2.462 Tera FLOPS.
But one huge assumption was implicitly made: that their useful work can be summed.
All the nodes are doing the same work actually, so the reality is that the useful computational power available on the whole Ethereum network is not far from what your computer or mine can provide. But that's the price to pay for trustless decentralization, and it's going to stay that way until true sharding is possible.
What is the computing power of the Ethereum network in FLOPS?
The dumb answer would be 0.
FLOPS stands for Floating Point Operation Per Seconds, Solidity does not support floating point operations, so the Ethereum network couldn't even manage 1 true FLOP if it wanted to.
Overall, Ethereum is very far from even coming close to what super computers are able to do. The way it is doing it however is (still) revolutionary.