This apocryphal quote was a statement about his overwhelming power (strong enough to hang people who have done no wrong), not on the mutability of the law. It is frequently mis-applied.
The quote is indeed about the law being a nose of wax, to borrow an old English phrase, and how with sympathetic enough courts almost any decision could be upheld. But it's nothing new, precisely the same crime can yield drastically different judgements depending on e.g. the defensive attorney's experience.