It's a page to describe what they do differently than PfSense, and that happens to be a) the web frontend and b) the choice of IDS/IPS. There is little reason to describe the "boring" standard stuff that's routing, firewalling, VPN server role that's the core of both projects. Have a look at the comparison table at the bottom for a better idea of the feature sets.

> A firewall is generally something you deploy along with a router. Does the term "firewall" just mean something else among the community of people repurposing old Dell towers to build complicated home networks?

That's an unfair characterization IMHO. In the commercial market, there are tons of "firewall appliances" that are used as router/firewall combinations: Sophos UTM, Watchguard boxes, Fortigate, ...

pfSense and OPNsense play in roughly the same space: a box you plug your WAN interface in that will do (primarily static) routing, firewalling with multiple zones, act as a VPN server. And typically have some amount of security checkboxes a la IDS, WAF, ..., although the commercial ones with the opertunity to sell subscriptions emphasize this more.