Without seeing the exact benches, I can’t give a guarantee, but the short answer is: it depends. If the wood was properly sealed with a quality outdoor sealant—like an oil-based deck sealer, spar varnish, or a penetrating wood oil—and the coating is still intact, the surface should stay fairly smooth for years. Sealed wood is less likely to soak up water, swell, dry out, and crack, which are the main things that lead to splintering.
That said, “sealed well” doesn’t mean sealed forever. Sun, rain, snow, temperature swings, and heavy use gradually wear down any finish. Once the sealant becomes thin or worn away, water can get into the grain. The wood expands and contracts, the surface fibers loosen, and that’s when you start feeling roughness or seeing small splinters. Softer woods like pine and fir tend to splinter sooner than dense woods like teak, cedar, or white oak.
If you can run your hand across the bench without catching on raised grain, the seal is probably doing its job. If the surface feels fuzzy, rough, or shows gray patches or cracks, it’s time to lightly sand and recoat it. For most park benches in normal outdoor exposure, recoating once a year or every two years is enough to keep them safe and smooth. In harsher climates or high-traffic areas, check them each spring.
So no, well-sealed benches should not splinter much over time, but they will eventually if the finish is ignored. A little maintenance goes a long way.