They source their claim to a WebPageTest run [1], and WPT does run scripts. I can reproduce their results as:
1. Open an incognito window in Chrome
2. Open devtools, and configure it as mobile
3. Navigate to reddit.com
4. See ~1.5MB transferred
[1] https://www.webpagetest.org/result/210707_AiDc36_6f01cdff93b...