The OMEMO authors have already responded to the point about the changelog, and it turns out the key length was always truncated like this (which is fine as Soatok themselves admit) and the change in the version they point out was only a slight wording change to emphasise this, not an actual spec change.

That Soatok jumps on this in their article without checking what the spec actually was in previous versions makes me think they didn’t really look very closely, but rather just looked for superficial support of their preconceived opinion.

As for post-quantum encryption: without knowing what quantum computers are really capable of, you can only speculate how to protect against them. The various proposals for that are highly debated and often turn out to be not any better or sometimes even worse than existing well established encryption methods.

Encryption is indeed a serious matter, as you say yourself. Peddling unproven and half-baked "post-quantum” encryption algorithms that might in fact lower and not higher protection against current and future attacks is not serious.

The serious response is to say we don’t know at the current time what can protect against possible future quantum computers and subsequentially minimize data retention and only use well proven state of the art encryption algorithms. Coincidentally XMPP is doing exactly that.