Since Google Play is not private, it should not be a big deal for a privacy company (Proton) to publish link to a Lumo Android .apk and a Github repository at https://lumo.proton.me/download and at https://protonapps.com (if that is official source). Also I would welcome F-Droid link.

There is a Github repo at https://github.com/ProtonLumo/android-lumo but the parent user shows no signs of validation as a official Proton AG account, despite Lumo claims there should be some check-mark on the user page.