The easiest way to get an IP rating is to use lots of glue.
If they went the o-ring route, it would take up more volume and would probably require replacement when the phone was opened as damage from dirt/age will make reusing the o-ring hard. Even worse is that resuse isn’t obvious to a layperson as it could be IP69 with a new o-ring and IP67 with reused o-ring.
It also has a headphone jack while being IP67 and is still reasonably thin, which is an insurmountable design feat according to some opinions I've seen here.
"User replaceable" barely does it justice either. I just had the battery in my hands in under 5 seconds easily. People earlier in the thread stating iPhone batteries are easily user replaceable are ridiculous, when the process can take upward of 20 minutes, and can leave the phone temporarily inoperable if any of the multiple connectors aren't quite seated correctly. I'm also intrigued as to where all this imaginary extra weight and bulk is coming from when implementing specifically the S5 battery enclosure design, because having stripped down hundreds of iPhones and dozens of Galaxy variants I just don't see it.
Making iPhone batteries user-replaceable means making the phone worse, heavier, larger, more expensive and with worse battery life.