You are both right and wrong. Yes, Oculink does have a different connector and no, it wouldn’t make sense for Framework or anyone else, to design an Expansion Card for said connector, because those are being connected via USB-C to the mainboard.
Oculink requires a direct PCI-E connection, realized by its connector either being included directly on the mainboard, or via an adapter card connected via either M.2 or PCI-E.
As there aren’t any laptops available with PCI-E slots, that leaves the M.2 connector, of which the Framework Laptop 13 only owns a single one, so you’d be forced to connect a different boot drive via USB if you occupy it with the Oculink adapter.
Due to its Expansion Bay system and the recently released Dual M.2 Adapter, the Framework Laptop 16 doesn’t have that issue, as has been tested by a community member: