From what I understand, when you find an airtag, you connect it to your device and it gives you the owners contact info. Anyone can get your phone number.
When you find an ottotag, you scan it and you contact the owner through ottomon. From here the GPS location is captured and communicated. All interactions are done through the service (ottomon.net). This layer act as a security layer where we filter out spam and possibly stalkers based on past behavior and other heuristics.
The two parties can communicate without revealing any information about themselves. So yeah, Apple could have made it much simpler, but they are my competitors now so let's keep this between us.
Edit: For more info -> https://www.ottomon.net/blog/privately-communicate-on-ottomo...