I happen to agree with the author simply based on the tight IAM integration between all AWS products. Working with GCP and manually processing OAuth2.0 tokens (three legged oauth, 2 legged oath, JSON web tokens, callbacks, etc.) with callbacks, signing/storing/expiring certificates is such a nightmare from an integrations perspective. IAM solves all of these problems. IAM alone is reason to go all in with AWS.
All I could mine from that was something like "don't do it because it's a lot of work." But, most things in life worth doing are hard work.
Very convincing...