I wonder if using an older puck affects charging speed on the Apple Watch S6.... Might be worth a test. I still have a 2m cable from the days of my S0, I may do a 'charge time' test compared to the puck that came with my S6.
I have also noticed that the older chargers are thicker than the newer ones.
To answer your question though, yes all genuine Apple Watch chargers work for all Apple Watches 🙂