Personally, I wouldn’t treat/seal anything that is going into my pond, just for the potential hazard it presents. Even if it says it’s non toxic, it really isn’t meant to be used in constant contact with water and designed to be safe or use in and around aquatic life, lastly, being out in the elements the sealer will last a year if your lucky before it needs to be reapplied, so to me not worth using it, even if it is truly non toxic.
The other thing is choose your wood wisely, don’t pick pieces that are rotten and falling apart, don’t build anything that wood is structurally holding back or up. Put the wood in aesthetically, natural looking spots around the pond, but also places that are easily accessed for removal if it rots faster than you thought. If you live long enough the pieces of wood you add to your pond will rot, no matter what you do to it, just the nature of wood.
Honestly, you are overthinking it, I have a lot of dead wood in my pond, off construction sites I’ve worked on, and hasnt rotted much at all over the years. Even if it does it is just wood that can be removed.