In our experience this is a very realistic value related to the amount of page views and other functionality offered by this plan.
It takes a pretty long time to reach 5 GB with a Discourse forum.
In other words: we see that this limit is almost never reached because if your forum is growing bigger than this, you already have had other reasons to upgrade to a higher plan, like custom plugins.
EDIT: I checked our numbers - Only 1.5 % of the servers on the Basic plan ever reached 5 GB.
So yes: seriously.