I'll start this by saying we build all content via Lightning. We've had Pardot for less than a year. Marketing builds a lot of templates for our sales team to use via Salesforce or Outlook to engage customers. When I go to Pardot Reports > Marketing Assets > Email > Templates the only email templates that show are the couple that were built in Classic when we were initially getting set up. We don't even use those templates. How do I view the templates that were built in Lightning here? Or how do I find the same reporting for Lightning email templates?
The report referenced, as you discovered, only applies to Pardot email templates. If you want to report on emails sent using the new lightning email builder, a salesforce admin needs to set up a custom salesforce report type to be able to aggregate the sending information that is stored on the email content object. This is not a “template” report per se, but a sent emails report, since you can now send an email with or without a template. You can aggregate by campaigns, however.
Here's a great guide on how to set up the custom report type:
https://greenkeydigital.com/custom-engagement-history-reports/