That's a bit of an exaggeration. Domain-validated certificates already existed, and were fairly commonplace, before Let's Encrypt. What was different about LE was that their certificates were free.

Let's not forget that it was mostly a manual process for most of us. Someone would obtain a cert, and email it to you, along with the private key. I'm sure old JIRA tickets would have so many "encrypted" zip files containing RSA 2048 keys and leaf certificates valid from 2008 to 2011.