I think even brilliant cryptographers can be wrong, or acting on strong biases formed from a limited perspective. In this case I think they have not thought about personal cryptographic key management and how people use tools like PGP in the wild well enough.
An engineer coming from a corporate world where everyone is comfortable with a model of centralized backups, centralized identity, and centralized trust is going to have a very different perspective than say, a Linux distribution maintainer, or those maintaining they core backbone of the internet.
I am closer to the latter camp, and obviously have my own biases here, but in spite of them I package Age in stagex, and we support it in keyfork, just so people have choices. Choices are always a good thing.
That said, it is my opinion that age does not even begin to approach the threat model or use cases PGP solves for. It does one thing, and it does not even do that thing as well as PGP does in most situations I can think of.
Just because someone is experienced in cryptography does not mean they have had significant exposure to environments where decentralized identity and trust are a hard requirement and where no alternatives to PGP exist, and where there is no customer service or IT team to bail you out, which really changes how we tend to think about these problems.
In my experience cryptography engineers that work on decentralized open source systems like Tor, blockchains, Linux distributions, etc, tend to strongly favor solutions like PGP as not "good" but the "least bad" option to avoid any single point of trust or failure.
Those that have spent their careers in the proprietary FAANG world tend to support using solutions like Fulcio or sigstore and using OIDC to let a central party sign for you with "keyless signing", which to me, is total nonsense. I assume anything that I cannot verify the integrity of for myself to be compromised.