Ken is a good friend in the industry and always has the best interest of email security at heart. This may have been an architectural oversight but they are not wrong that SPF is surely a cause for concern, as is misconfigured DNS based trust and recertifications via arc (which was supposed to solve a problem for forwarding scenarios).

The centralization of email services to a handful of providers basically has led to multihoming of millions of domains that open SPF auth to the same handful. Any integrations by them or changes to existing stack can cause issues to pop up, because delegation of sending rights isn't strictly auth controlled. The same also happens with dkim delegation to saas providers who share backend keys across other customers of theirs and if their API is open to experiment (or an account gets popped) then the customer domains are possibly at risk.

Email is hard to do right. No auth no entry should be the default. But majority of domain owners aren't very good at figuring out how to secure things, or have business/product interests that are a priority, specially when delegated and authorized to third party senders on their behalf.