From the page you link (and in which I see nowhere the author designates themself a "cryptography expert", albeit they may be one):
> The main reason I said to avoid Session, all those months ago, was simply due to their decision to remove forward secrecy
I often see this complaint. But it seems to me that, since Session allows users to generate a new private key arbitrarily on the device (and without even associating it with a phone number on a central server) it benefits less from the feature.
> Forward secrecy protects past sessions against future compromises of keys or passwords. By generating a unique session key for every session a user initiates, the compromise of a single session key will not affect any data other than that exchanged in the specific session protected by that particular key.[0]