Optical (Toslink) is a digital interface; turntable signals from the cartridge motor (a cartridge generates it's own AC voltage signal) are analog, as is the input to the RIAA filter / small-signal amplifier in a preamp.
An HDMI cable could be used with a turntable, but isn't. There are critical parameters that must be addressed between the cartridge and phono preamp, in particular minimal capacitance, that HDMI cables are not designed to address.
In other words, although you could, with some DIY'ing with a Dremel and some soldering, make the necessary mating connectors fit, and then use an HDMI cable, you would not be using the best connection and probably would be making a poor connection that fails to address the electrical requirements needed for optimum signal transfer.
Specifically, any excess capacitance would roll off the High Frequency response of your cartridge.
As it turns out, you have asked about a connection that has unique conditions; the Cartridge to Phono Preamp interface is special because of the extremely small voltage signals involved.
Remember there are no dumb questions so don't fret over asking anything you are curious about; you will always learn something.
