Nissenjuyon

However, the beard of Hemill was not invented by Disney...

Actually, it was Disney. The real, proper, most canonical Luke Skywalker didn't wear a beard.

But why did he act then? I don't think they hid the script from him until the very last dub, or pointed a gun at him.

As far as I know, he could have signed a contract for all three films from the very beginning, when the sequels were just being planned. And when he found out what role he would have to play, it would be difficult to break the contract and ditch Disney.
Or maybe he didn't care enough about the character to make a stand and quit, he just did his part and that's it. Or the money won. There are many reasons. But I still respect him for this quote, that he didn't try to bullshit and say how great his character was developed, but honestly said that it was just bad.