He didn't send out resume to apply for the job, jobs found him via blog post.

How does he know it came from the blog and not his LinkedIn profile or github page?

I get 3 or 4 emails a day for jobs without having a blog, so I'm curious to understand how he can tell that his blog is what is drawing the attention.

I usually get email like "We found you via github! If you are seeking for job, please apply today blah blah blah". Maybe these recruiters mention his website?

I don't have linkedIn, and I usually get messages after an article got some attention on social networks :)

Also, sometimes, my website is directly mentioned in the emails.