A water resistant watch does not have to glue everything together. Just look at all the mechanical watches out there.
A mechanical watch -- or, heck, any traditional quartz-powered watch -- also doesn't have heart rate sensors inside the caseback.But you can't assume it doesn't Need "Glue." A smart Watch and mechanical Watch have completely different internal parts, Ingress points and not to mention, what other smart Watch uses screws entirely to bond the components together? It's a likely a cheaper method as well, Because it's a piece of technology, not a mechanical Watch that requires service after so many years.
Now, yes, there are different methods that traditional watches use to secure their backs, whether it's a few small screws holding it in place, or it just snaps in, or the entire back screws down in one piece.
What I want to know is exactly how the Series 2 caseback is secured to the body. The teardown at iFixit.com barely mentioned it at all, and didn't say whether it was snapped in or simply glued.