Does JMeter end up testing the back-end of a web-application with the front-end as an entry point?

I'm told that we need to do some performance testing on one of our web-applications, so I'm trying to get some JMeter stuff to work, which as far as I know would simulate the HTTP GETS and POSTS. However, one of my colleagues is telling me that if I use it, it'd only accomplish FE testing. But, if I do this, it still is able to create items in the database and interact with the logic, so I figured it should be sufficient for performance testing of the back-end. Her reasoning is that "if it goes through http pages, we can’t tell which affects the performance".

So am I totally wrong? I'm confused.