The post is about how webhook requests are usually signed and api responses are not.
For me it seems clear that the reason for this different approach is that api requests are already authenticated. Signing them would yield little additional security. Diminishing returns like the debate over long lived (manually refreshed) api keys versus short lived access tokens with long lived refresh tokens - or, annoyingly, single use refresh tokens that you have to keep track of along with the access token.
Webhooks are unauthenticated post requests that anyone could send if they know the receiving url, so they inherently need sender verification.
For me it seems clear that the reason for this different approach is that api requests are already authenticated. Signing them would yield little additional security. Diminishing returns like the debate over long lived (manually refreshed) api keys versus short lived access tokens with long lived refresh tokens - or, annoyingly, single use refresh tokens that you have to keep track of along with the access token.
Webhooks are unauthenticated post requests that anyone could send if they know the receiving url, so they inherently need sender verification.