They always do this. They do release the code and keep their promise a little later and not immediately after release of their newest product. This is actually a well known thing Proton does with how it open sources things eventually. Proton Authenticator appears to be an outlier here. AI/Lumo is a bigger product so they may be ironing some things out. But I have no reason to doubt it is not indeed open source in spirit today. They just have to make it so.
This is more a non issue for me but I do see your point too. They should ideally, in a perfect world, release it all when they release the product - but such is not the case.