Satoshi did know about it. Quantum was a thing before year 2000. Don't tell me the man who created Bitcoin does not know what quantum is.
If we look at the history of quantum computers, it started around 1980's, so definitely, Satoshi have known and think about quantum computers because it is already existing and developing even before Bitcoin was created.
Re: Did Satoshi think that quantum computers will exist?
He also knew that "second generation quantum computers" will exist.
And for the "second generation quantum computers" people are already developing post SHA-hash signature systems. So we would then change to post SHA-hash signature systems before "second generation quantum computers" exist.
The development of "second generation quantum computers" will bring the most exciting times, hashing will be history. We will make all "lost" coins active.