Free plan is fine if the video files are part of a website hosted by Cloudflare. The streaming service doesn’t use use S3. For a more specific and official answer, you’ll have to contact Support:
Login to Cloudflare and then contact Cloudflare Support
2.8 Limitation on Non-HTML Caching
The Service is offered primarily as a platform to cache and serve web pages and websites. Unless explicitly included as a part of a Paid Service purchased by you, you agree to use the Service solely for the purpose of serving web pages as viewed through a web browser or other application and the Hypertext Markup Language (HTML) protocol or other equivalent technology. Use of the Service for the storage or caching of video (unless purchased separately as a Paid Service) or a disproportionate percentage of pictures, audio files, or other non-HTML content, is prohibited.