They said they had access to my computer. As proof, they were sending the email from my own account, and sure enough the email had been sent from my account.
Finally, as proof that they weren't messing around, they attached part of the video to the email, which I could download and open and use to verify that they really did have me.
Imagine being someone that uses the internet but doesn't understand email, who had actually done the things they described:
1. Someone sent an email describing your illegal act.
2. They proved they have control of your computer by sending you an email from your account.
3. They sent part of the video. Downloading. Installing. Why's there no video?
In my case, it also contained a password I had only ever used for a LiveJournal account I had forgotten about and not logged into for several years by that point.
[0] https://kitsunesoftware.wordpress.com/2018/08/09/anatomy-of-...
I'm guessing there are many people who've been doing stuff online they'd worry about their friends knowing, who would pay.
Fwiw,I checked haveibeenpwned with him, and we found where they would have got the password from. My spam filter had caught lots of emails with identical wording, so it was reassuring for him to see the same emails, understand how it happened, and learn some password management stuff.