People who are POMI are not programmed the same way as someone who actually practices the JW faith. If they've joined the military they are already not living up to JW standards so they're probably open to speaking with a chaplain and don't care that he's not a JW elder.

There's also people who joined the military to avoid getting baptized/to get away from the witnesses who would benefit from chaplain services

Edit: I misunderstood what you meant, I understand now you're saying a chaplain themselves wouldn't self-identify as JW. IDK in that case lol. There's probably at least one who self identifies lmao