As in not-too-large can of a beer, as it is limited to no more than 2 users and 1000 invoices.

It isn't free as in software, as both FSF and OSI requires software compatible with their goals to have no limitations on use.

Meh, this feels more like marketing than open source. The "free" on premise hosted version is limited. Also, no mention of "apps", like inventory, stripe, PoS, etc. for the self hosted version.

Edit: it seems the on-premise version supports apps: There is no app included in any plan. All apps must be purchased separately from the App Store.