So how secure are satoshi's coins which were mined directly and put in a public address instead of a BTC address.

Because you are saying that reusing addresses is not good because its not as secure but if this was the case why wouldn't this person who is solving all these puzzles for less than <1 BTC instead try and crack Satoshi's early coins which are sitting in all those public key addresses?

I think why its faster is because the search space is much small compared to the entire 128 bit search space, so its still safe to re-use addresses since as far as we know ECDSA hasn't been cracked yet or some exploit found.