Can’t speak to this exact circumstance, but more generally: The ONT translates the SFP+ networking to fibre optic, but the modem is still somewhat necessary for logins if you use PPPoE as a wrapper for example. In telecom fibre optic, it often also assigns a particular vlan to internet packets and separate vlans for TV and phone. But I’m not an expert here, just explaining why I needed a modem function in my router as well as a media converter to house the ONT.
As far as I know, nobody uses separate boxes for the modem and router, that kind of thinking died when wifi became more widespread and included by default with ISP plans.
The component that does the PPPoE and VoIP VLAN is typically just referred to as a router (or a "residential gateway" for companies that want to sound less technical), I've never heard as it referred to as a modem, usually the ONT is referred to as a modem as it's MODulating/DEModulating the optical signal.
I wouldn't really call that a "modem" though, it's not really doing modulation/demodulation work to convert between media types. The terminology I usually hear for the provider's box handling any final authentication and VLAN splitting is usually a "residential gateway", which can be configured to bridge to a client's equipment.
Definitely splitting hairs here though on terminology.