@BeAware You saw that account, too?
In Linus’s case, he could put his Mastodon handle in the Linux kernel’s comments, or post it to the kernel mailing list. Of course, not everyone can do that.
Some people doing Reddit AMA’s have taken a photo of themselves holding a piece of paper with today’s date and the subreddit name. Perhaps that can be a low-tech verification solution.
Or, have someone post something on another social media site, “I’m on Mastodon at…” Again, not ideal, but doable.