In the context of their blog post, it's assumed that the reader has some knowledge of web technologies — if you can print data to your own webpage, you can also

In the context of their blog post, it's assumed that the reader has some knowledge of web technologies — if you can print data to your own webpage, you can also exfiltrate it. It's not reasonable to expect the author to explain _every_ underlying technology involved since that was likely not the scope of the post.