Well, it is a decade-and-a-half old and fairly common in the Python web world. I don't think there's any shame in not knowing what it is if you haven't encountered it, but I also don't think there's any shame in assuming a developer audience reading about web development would at least know how to figure out what it is.

Everyone on HN here has heard of Jinja...

Apart from the person who has "fullstack" in his username.

I know of Jinja only because I have dabbled in some Ansible and Python. Had I not done either I probably would have been none the wiser too.

When I look at the page, there’s a link on the word Jinja. And the next word is templates. Maybe you aren’t familiar with what a template is, but they are hardly obscure. Here’s a list of large websites that used a Perl-based templating engine a decade ago: http://www.masonhq.com/sites

There is literally a link on the word Jinja to the Jinja docs...