He tried to use the same wallet.dat, I don't know if you're supposed to do that.
yes, & why can't?, i guess it should work.
Firstly, I feel obliged to ask everyone to read
the precautions about the use of Electron Cash before proceed,
Secondly, my experience with full clients:
I started with installation of the latest version of BitcoinABC - 0.14.6
Than I shut it down, and run Bitcoin Core, no problems occurred, it took the same AppData directory and the encrypted wallet.dat in it.
So if there's a problem with compatibility - follow
importprivkey way.
... but how can yobit and other did provided BCC to all the btc holders on their exchanges if it isn't that simple ?
They shut down BTC deposit/withdrawal. And then distribute inner, "paper" token. It's quite similar, I think, to what HitBTC firstly did in spring with Classic tokens.