Are Fiber Media Converters Technically Modems?
All modems are PHYs (physical layers) in the OSI network model, but only some PHYs are complex enough to be considered modems. Why? I think it depends on four things:


simple or complex modulation
carrier or baseband
WAN or local
historical use


For example, an Ethernet PHY uses simple encoding depending on the speed in use (MLT, PAM). These schemes have a fairly small number of symbols / constellation points, and they're baseband encoded. So is Ethernet a modem? No, at least for 10/100/1000-base types.

Yet as they find more creative ways to squeeze bits out of copper and other media to push Ethernet ever faster, the complexity line between Ethernet 'phy' and 'modem' becomes increasingly blurred. Nevertheless, since Ethernet is a LAN connection only, it's not a modem in that sense. So, not a modem.

A cable modem on the other hand uses QAM modulation which has a large constellation: 64, 256, 1024 or more points. It's a WAN connection too, part of a service network, so it resembles a historical modem in that regard. And it uses carrier frequencies. So it clearly ticks all three boxes: complex, WAN, and uses carrier. So, modem from day one.

How about MoCA? It very definitely uses carrier frequencies with an OFDM-based modulation scheme, but nevertheless isn't considered a modem at least in general usage (generally the term 'adapter' is used for MoCA.) Why? It's LAN only.

Finally, the optical example you show - a media converter - seems to be only changing the electrical to optical and vice-versa, and only in a local point-to-point connection. It’s not doing any significant re-encoding to / from complex modulation scheme, and it's not connecting to a WAN. So, not a modem.