It is unlikely that Meta is able to decrypt the messages on their side. The WhatsApp desktop client is actually very crippled by the fact that it can’t just ask someone for the messages, it is another participant in end to end encryption and needs the shared keys. If they would defeat end to end encryption they would first implement the desktop client in a more functional and easy way.

But it is possible for the client itself to build a map of advertising id -> interests and send that over to meta separately. This would be similar to one of chrome's proposals.