I don't want to assume anything about your work history, but I have worked at 2 fortune 500 companies, and I can absolutely say that if its not in writing it didn't happen.
Perhaps these companies were outliers, and I don't think anything I experience was a result of malice. However, when the higher ups are looking for answers, middle management tends to put blame on the developers. I can't tell you how many times I had to dig up old email chains to essentially prove what was communicated. People forget, people misremember, and people will fabricate to avoid repercussions. I have found this to be a universal truth of business, so I can see why someone would say your comment is naive.
Also like the comment above yours states, there is literally no downside to re-affirming the discussed points. I've had instances where once things were put in writing, my manager noticed something that wasn't clear and we were able to circumvent issues.